RFC: 791 INTERNET PROTOCOL DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION September 1981 prepared for Defense Advanced Research Projects Agency Information Processing Techniques Office 1400 Wilson Boulevard Arlington, Virginia 22209 by Information Sciences Institute University of Southern California 4676 Admiralty Way Marina del Rey, California 90291 September 1981 Internet Protocol TABLE OF CONTENTS PREFACE ........................................................ iii 1. INTRODUCTION ..................................................... 1 1.1 Motivation .................................................... 1 1.2 Scope ......................................................... 1 1.3 Interfaces .................................................... 1 1.4 Operation ..................................................... 2 2. OVERVIEW ......................................................... 5 2.1 Relation to Other Protocols ................................... 9 2.2 Model of Operation ............................................ 5 2.3 Function Description .......................................... 7 2.4 Gateways ...................................................... 9 3. SPECIFICATION ................................................... 11 3.1 Internet Header Format ....................................... 11 3.2 Discussion ................................................... 23 3.3 Interfaces ................................................... 31 APPENDIX A: Examples & Scenarios ................................... 34 APPENDIX B: Data Transmission Order ................................ 39 GLOSSARY ............................................................ 41 REFERENCES .......................................................... 45 [Page i] September 1981 Internet Protocol [Page ii] September 1981 Internet Protocol PREFACE This document specifies the DoD Standard Internet Protocol. This document is based on six 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 aspects of addressing, error handling, option codes, and the security, precedence, compartments, and handling restriction features of the internet protocol. Jon Postel Editor [Page iii] September 1981 RFC: 791 Replaces: RFC 760 IENs 128, 123, 111, 80, 54, 44, 41, 28, 26 INTERNET PROTOCOL DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION 1. INTRODUCTION 1.1. Motivation The Internet Protocol is designed for use in interconnected systems of packet-switched computer communication networks. Such a system has been called a "catenet" [1]. The internet protocol provides for transmitting blocks of data called datagrams from sources to destinations, where sources and destinations are hosts identified by fixed length addresses. The internet protocol also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through "small packet" networks. 1.2. Scope The internet protocol is specifically 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 augment end-to-end data reliability, flow control, sequencing, or other services commonly found in host-to-host protocols. The internet protocol can capitalize on the services of its supporting networks to provide various types and qualities of service. 1.3. Interfaces This protocol is called on by host-to-host protocols in an internet environment. This protocol calls on local network protocols to carry the internet datagram to the next gateway or destination host. For example, a TCP module would call on the internet module to take a TCP segment (including the TCP header and user data) as the data portion of an internet datagram. The TCP module would provide the addresses and other parameters in the internet header to the internet 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 case, for example, the internet module would call on a [Page 1] September 1981 Internet Protocol Introduction 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. 1.4. Operation The internet protocol implements two basic functions: addressing and fragmentation. 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) 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. 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 datagram. The Time to Live is an indication of an upper bound on 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 destination, the internet datagram is destroyed. The time to live can be thought of as a self destruct time limit. [Page 2] September 1981 Internet Protocol Introduction The Options provide for control functions needed or useful in some situations but unnecessary for the most common communications. The options include provisions for timestamps, security, 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. Errors detected may be reported via the Internet Control Message Protocol (ICMP) [3] which is implemented in the internet protocol module. [Page 3] September 1981 Internet Protocol [Page 4] September 1981 Internet Protocol 2. OVERVIEW 2.1. Relation to Other Protocols The following diagram illustrates the place of the internet protocol in the protocol hierarchy: +------+ +-----+ +-----+ +-----+ |Telnet| | FTP | | TFTP| ... | ... | +------+ +-----+ +-----+ +-----+ | | | | +-----+ +-----+ +-----+ | TCP | | UDP | ... | ... | +-----+ +-----+ +-----+ | | | +--------------------------+----+ | Internet Protocol & ICMP | +--------------------------+----+ | +---------------------------+ | Local Network Protocol | +---------------------------+ Protocol Relationships Figure 1. Internet protocol interfaces on one side to the higher level host-to-host protocols and on the other side to the local network protocol. In this context a "local network" may be a small network in a building or a large network such as the ARPANET. 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 the destination address and other parameters as arguments of the call. The internet module prepares a datagram header and attaches the data to it. The internet module determines a local network address for this internet address, in this case it is the address of a gateway. [Page 5] September 1981 Internet Protocol Overview 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 it, then sends the result via the local network. The datagram arrives at a gateway host wrapped in the local network header, 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 is to 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 call, passing the source address and other parameters as results of the call. Application Application Program Program \ / Internet Module Internet Module Internet Module \ / \ / LNI-1 LNI-1 LNI-2 LNI-2 \ / \ / Local Network 1 Local Network 2 Transmission Path Figure 2 [Page 6] September 1981 Internet Protocol Overview 2.3. 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. Thus, 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 names, addresses, and routes [4]. 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 internet 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 network number, followed by local address (called the "rest" field). There are three formats or classes of internet addresses: in class a, the high order bit is zero, the next 7 bits are the network, and the last 24 bits are the local address; in class b, the high order two bits are one-zero, the next 14 bits are the network and the last 16 bits are the local address; in class c, the high order three bits are one-one-zero, the next 21 bits are the network and the last 8 bits are the local address. 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. Some hosts will also have several physical interfaces (multi-homing). That is, provision must be made for a host to have several physical interfaces to the network with each having several logical internet addresses. [Page 7] September 1981 Internet Protocol Overview Examples of address mappings may be found in "Address Mappings" [5]. Fragmentation Fragmentation of an internet datagram is necessary when it originates in a local net that allows a large packet size and must 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 [6]. 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 placed in the first new internet datagram, and the total length field is set to the length of the first [Page 8] September 1981 Internet Protocol Overview 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 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 of an internet datagram, an internet protocol module (for example at a destination host) combines internet datagrams 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. 2.4. Gateways Gateways implement internet protocol to forward datagrams between networks. Gateways also implement the Gateway to Gateway Protocol (GGP) [7] to coordinate routing and other internet control information. In a gateway the higher level protocols need not be implemented and the GGP functions are added to the IP module. +-------------------------------+ | Internet Protocol & ICMP & GGP| +-------------------------------+ | | +---------------+ +---------------+ | Local Net | | Local Net | +---------------+ +---------------+ Gateway Protocols Figure 3. [Page 9] September 1981 Internet Protocol [Page 10] September 1981 Internet Protocol 3. SPECIFICATION 3.1. Internet Header Format A summary of the contents of the internet header follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Example Internet Datagram Header Figure 4. Note that each tick mark represents one bit position. Version: 4 bits The Version field indicates the format of the internet header. This document describes version 4. IHL: 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 correct header is 5. [Page 11] September 1981 Internet Protocol Specification 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 (generally by accepting only traffic above a certain precedence at time of high load). The major choice is a three way tradeoff between low-delay, high-reliability, and high-throughput. Bits 0-2: Precedence. Bit 3: 0 = Normal Delay, 1 = Low Delay. Bits 4: 0 = Normal Throughput, 1 = High Throughput. Bits 5: 0 = Normal Relibility, 1 = High Relibility. Bit 6-7: Reserved for Future Use. 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+-----+-----+-----+-----+ | | | | | | | | PRECEDENCE | D | T | R | 0 | 0 | | | | | | | | +-----+-----+-----+-----+-----+-----+-----+-----+ Precedence 111 - Network Control 110 - Internetwork Control 101 - CRITIC/ECP 100 - Flash Override 011 - Flash 010 - Immediate 001 - Priority 000 - Routine The use of the Delay, Throughput, and Reliability indications may increase the cost (in some sense) of the service. In many networks better performance for one of these parameters is coupled with worse performance on another. Except for very unusual cases at most two of these three indications should be set. The type of service is used to specify the treatment of the datagram during its transmission through the internet system. Example mappings of the internet type of service to the actual service provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET is given in "Service Mappings" [8]. [Page 12] September 1981 Internet Protocol Specification The Network Control precedence designation is intended to be used within a network only. The actual use and control of that designation is up to each network. The Internetwork Control designation is intended for use by gateway control originators only. If the actual use of these precedence designations is of concern to a particular network, it is the responsibility of that network to control the access to, and use of, those precedence designations. Total Length: 16 bits Total Length is the length of the datagram, measured in octets, including internet header and data. This field allows the length of a datagram to 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 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 1: (DF) 0 = May Fragment, 1 = Don't Fragment. Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments. 0 1 2 +---+---+---+ | | D | M | | 0 | F | F | +---+---+---+ Fragment Offset: 13 bits This field indicates where in the datagram this fragment belongs. [Page 13] September 1981 Internet Protocol Specification 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 in the internet system. If this field contains the value zero, then the datagram must be destroyed. This field is modified in internet header processing. The time is measured in units of seconds, but since every module that processes a datagram must decrease the TTL by at least one even if it process the datagram in less than a second, the TTL must be thought of only as an upper bound on the time a datagram may exist. The intention is to cause undeliverable datagrams to be discarded, and to bound the maximum datagram lifetime. 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 "Assigned Numbers" [9]. Header Checksum: 16 bits A checksum on the header only. Since some header fields 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. See section 3.2. Destination Address: 32 bits The destination address. See section 3.2. [Page 14] September 1981 Internet Protocol Specification Options: variable The options may appear or not in datagrams. They must be implemented by all IP modules (host and gateways). What is optional is their transmission in any particular datagram, not their implementation. In some environments the security option may be required in all datagrams. The option field is variable in length. There may be zero or more options. There are two cases for the format of an option: Case 1: A single octet of option-type. Case 2: 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 copied flag, 2 bits option class, 5 bits option number. The copied flag indicates that this option is copied into all fragments on fragmentation. 0 = not copied 1 = copied The option classes are: 0 = control 1 = reserved for future use 2 = debugging and measurement 3 = reserved for future use [Page 15] September 1981 Internet Protocol Specification The following internet options are defined: CLASS NUMBER LENGTH DESCRIPTION ----- ------ ------ ----------- 0 0 - End of Option list. This option occupies only 1 octet; it has no length octet. 0 1 - No Operation. This option occupies only 1 octet; it has no length octet. 0 2 11 Security. Used to carry Security, Compartmentation, User Group (TCC), and Handling Restriction Codes compatible with DOD requirements. 0 3 var. Loose Source Routing. Used to route the internet datagram based on information supplied by the source. 0 9 var. Strict Source Routing. Used to route the internet datagram based on information supplied by the source. 0 7 var. Record Route. Used to trace the route an internet datagram takes. 0 8 4 Stream ID. Used to carry the stream identifier. 2 4 var. Internet Timestamp. Specific Option Definitions End of Option List +--------+ |00000000| +--------+ 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, or for any other reason. [Page 16] September 1981 Internet Protocol Specification No Operation +--------+ |00000001| +--------+ Type=1 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, or for any other reason. Security This option provides a way for hosts to send security, compartmentation, handling restrictions, and TCC (closed user group) parameters. The format for this option is as follows: +--------+--------+---//---+---//---+---//---+---//---+ |10000010|00001011|SSS SSS|CCC CCC|HHH HHH| TCC | +--------+--------+---//---+---//---+---//---+---//---+ Type=130 Length=11 Security (S field): 16 bits Specifies one of 16 levels of security (eight of which are reserved for future use). 00000000 00000000 - Unclassified 11110001 00110101 - Confidential 01111000 10011010 - EFTO 10111100 01001101 - MMMM 01011110 00100110 - PROG 10101111 00010011 - Restricted 11010111 10001000 - Secret 01101011 11000101 - Top Secret 00110101 11100010 - (Reserved for future use) 10011010 11110001 - (Reserved for future use) 01001101 01111000 - (Reserved for future use) 00100100 10111101 - (Reserved for future use) 00010011 01011110 - (Reserved for future use) 10001001 10101111 - (Reserved for future use) 11000100 11010110 - (Reserved for future use) 11100010 01101011 - (Reserved for future use) [Page 17] September 1981 Internet Protocol Specification Compartments (C field): 16 bits An all zero value is used when the information transmitted is not compartmented. Other values for the compartments field may be obtained from the Defense Intelligence Agency. Handling Restrictions (H field): 16 bits The values for the control and release markings are alphanumeric digraphs and are defined in the Defense Intelligence Agency Manual DIAM 65-19, "Standard Security Markings". Transmission Control Code (TCC field): 24 bits Provides a means to segregate traffic and define controlled communities of interest among subscribers. The TCC values are trigraphs, and are available from HQ DCA Code 530. Must be copied on fragmentation. This option appears at most once in a datagram. Loose Source and Record Route +--------+--------+--------+---------//--------+ |10000011| length | pointer| route data | +--------+--------+--------+---------//--------+ Type=131 The loose source and record route (LSRR) 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, and to record the route information. 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, the pointer octet, and length-3 octets of route data. The third octet is the pointer into the route data indicating the octet which begins the next source address to be processed. The pointer is relative to this option, and the smallest legal value for the pointer is 4. A route data is composed of a series of internet addresses. Each internet address is 32 bits or 4 octets. If the pointer is greater than the length, the source route is empty (and the recorded route full) and the routing is to be based on the destination address field. [Page 18] September 1981 Internet Protocol Specification If the address in destination address field has been reached and the pointer is not greater than the length, the next address in the source route replaces the address in the destination address field, and the recorded route address replaces the source address just used, and pointer is increased by four. The recorded route address is the internet module's own internet address as known in the environment into which this datagram is being forwarded. This procedure of replacing the source route with the recorded route (though it is in the reverse of the order it must be in to be used as a source route) means the option (and the IP header as a whole) remains a constant length as the datagram progresses through the internet. This option is a loose source route because the gateway or host IP is allowed to use any route of any number of other intermediate gateways to reach the next address in the route. Must be copied on fragmentation. Appears at most once in a datagram. Strict Source and Record Route +--------+--------+--------+---------//--------+ |10001001| length | pointer| route data | +--------+--------+--------+---------//--------+ Type=137 The strict source and record route (SSRR) 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, and to record the route information. 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, the pointer octet, and length-3 octets of route data. The third octet is the pointer into the route data indicating the octet which begins the next source address to be processed. The pointer is relative to this option, and the smallest legal value for the pointer is 4. A route data is composed of a series of internet addresses. Each internet address is 32 bits or 4 octets. If the pointer is greater than the length, the source route is empty (and the [Page 19] September 1981 Internet Protocol Specification recorded route full) and the routing is to be based on the destination address field. If the address in destination address field has been reached and the pointer is not greater than the length, the next address in the source route replaces the address in the destination address field, and the recorded route address replaces the source address just used, and pointer is increased by four. The recorded route address is the internet module's own internet address as known in the environment into which this datagram is being forwarded. This procedure of replacing the source route with the recorded route (though it is in the reverse of the order it must be in to be used as a source route) means the option (and the IP header as a whole) remains a constant length as the datagram progresses through the internet. This option is a strict source route because the gateway or host IP must send the datagram directly to the next address in the source route through only the directly connected network indicated in the next address to reach the next gateway or host specified in the route. Must be copied on fragmentation. Appears at most once in a datagram. Record Route +--------+--------+--------+---------//--------+ |00000111| length | pointer| route data | +--------+--------+--------+---------//--------+ Type=7 The record 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, the pointer octet, and length-3 octets of route data. The third octet is the pointer into the route data indicating the octet which begins the next area to store a route address. The pointer is relative to this option, and the smallest legal value for the pointer is 4. A recorded route is composed of a series of internet addresses. Each internet address is 32 bits or 4 octets. If the pointer is [Page 20] September 1981 Internet Protocol Specification greater than the length, the recorded route data area is full. The originating host must compose this option with a large enough route data area to hold all the address expected. The size of the option does not change due to adding addresses. The intitial contents of the route data area must be zero. When an internet module routes a datagram it checks to see if the record 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 recorded route begining at the octet indicated by the pointer, and increments the pointer by four. If the route data area is already full (the pointer exceeds the length) the datagram is forwarded without inserting the address into the recorded route. If there is some room but not enough room for a full address to be inserted, the original datagram is considered to be in error and is discarded. In either case an ICMP parameter problem message may be sent to the source host [3]. Not copied on fragmentation, goes in first fragment only. Appears at most once in a datagram. Stream Identifier +--------+--------+--------+--------+ |10001000|00000010| Stream ID | +--------+--------+--------+--------+ Type=136 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. Appears at most once in a datagram. [Page 21] September 1981 Internet Protocol Specification Internet Timestamp +--------+--------+--------+--------+ |01000100| length | pointer|oflw|flg| +--------+--------+--------+--------+ | internet address | +--------+--------+--------+--------+ | timestamp | +--------+--------+--------+--------+ | . | . . Type = 68 The Option Length is the number of octets in the option counting the type, length, pointer, and overflow/flag octets (maximum length 40). The Pointer is the number of octets from the beginning of this option to the end of timestamps plus one (i.e., it points to the octet beginning the space for next timestamp). The smallest legal value is 5. The timestamp area is full when the pointer is greater than the length. The Overflow (oflw) [4 bits] is the number of IP modules that cannot register timestamps due to lack of space. The Flag (flg) [4 bits] values are 0 -- time stamps only, stored in consecutive 32-bit words, 1 -- each timestamp is preceded with internet address of the registering entity, 3 -- the internet address fields are prespecified. An IP module only registers its timestamp if it matches its own address with the next specified internet address. The Timestamp is a right-justified, 32-bit timestamp in milliseconds since midnight UT. If the time is not available in milliseconds or cannot be provided with respect to midnight UT then any time may be inserted as a timestamp provided the high order bit of the timestamp field is set to one to indicate the use of a non-standard value. The originating host must compose this option with a large enough timestamp data area to hold all the timestamp information expected. The size of the option does not change due to adding [Page 22] September 1981 Internet Protocol Specification timestamps. The intitial contents of the timestamp data area must be zero or internet address/zero pairs. If the timestamp data area is already full (the pointer exceeds the length) the