By RFC org•September 5, 2025•

rfc

rfc791

RFC 791 - Internet Protocol

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

PREFACE ........................................................ iii

1. Introduction

1.1 Motivation .................................................... 1 1.2 Scope ......................................................... 1 1.3 Interfaces .................................................... 1 1.4 Operation ..................................................... 2

2. Overview

2.1 Relation to Other Protocols ................................... 9 2.2 Model of Operation ............................................ 5 2.3 Function Description .......................................... 7 2.4 Gateways ...................................................... 9

3. Specification

3.1 Internet Header Format ....................................... 11 3.2 Discussion ................................................... 23 3.3 Interfaces ................................................... 31

Appendix A: Examples & Scenarios

Appendix B: Data Transmission Order

Glossary

References

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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
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September 1981
RFC: 791
Replaces: RFC 760
IENs 128, 123, 111, 80, 54, 44, 41, 28, 26

1.1. Motivation

Internet Protocol
DARPA INTERNET PROGRAM
PROTOCOL SPECIFICATION

1. Introduction

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
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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.
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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.
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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.
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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
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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.
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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
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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.
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3. Specification

3.1. Internet Header Format

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.
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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].
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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.
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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.
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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
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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.
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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)
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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.

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.
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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.

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

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

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.

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
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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.
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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
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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 datagram is forwarded without inserting the
timestamp, but the overflow count is incremented by one.

If there is some room but not enough room for a full timestamp
to be inserted, or the overflow count itself overflows, 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]. The timestamp option is not copied upon fragmentation. It is carried in the first fragment. Appears at most once in a datagram. 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 must be conservative in its sending behavior, and liberal in its receiving behavior. That is, it must be careful to send well-formed datagrams, but must 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.
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Addressing

To provide for flexibility in assigning address to networks and

allow for the large number of small to intermediate sized networks

the interpretation of the address field is coded to specify a small
number of networks with a large number of host, a moderate number of networks with a moderate number of hosts, and a large number of networks with a small number of hosts. In addition there is an escape code for extended addressing mode. Address Formats:

High Order Bits Format Class
--------------- ------------------------------- ---- 0 7 bits of net, 24 bits of host a 10 14 bits of net, 16 bits of host b 110 21 bits of net, 8 bits of host c 111 escape to extended addressing mode A value of zero in the network field means this network. This is only used in certain ICMP messages. The extended addressing mode is undefined. Both of these features are reserved for future use.

The actual values assigned for network addresses is given in
"Assigned Numbers" [9]. The local address, assigned by the local network, must allow for a single physical host to act as several distinct internet hosts. That is, there must be a mapping between internet host addresses and

network/host interfaces that allows several internet addresses to
correspond to one interface. It must 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 "Address Mappings" [5]. 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
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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 (of course, the header is counted in the total length and not in the fragments). 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. The fields which may be affected by fragmentation include: (1) 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 permitted, 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. One example of use of the Don’t Fragment feature is to down line load a small host. A small host could have a boot strap program that accepts a datagram stores it in memory and then executes it.

The fragmentation and reassembly procedures are most easily
described by examples. The following procedures are example implementations. 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 4, 5, and 6 (but not 7).
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An Example Fragmentation Procedure

The maximum sized datagram that can be transmitted through the
next network is called the maximum transmission unit (MTU).

If the total length is less than or equal the maximum transmission

unit then submit this datagram to the next step in datagram
processing; otherwise cut the datagram into two fragments, the first fragment being the maximum size, and the second fragment being the rest of the datagram. The first fragment is submitted to the next step in datagram processing, while the second fragment is submitted to this procedure in case it is still too large. Notation:

FO - Fragment Offset

IHL - Internet Header Length
DF - Don’t Fragment flag

MF - More Fragments flag

TL - Total Length

OFO - Old Fragment Offset

OIHL - Old Internet Header Length

OMF - Old More Fragments flag

OTL - Old Total Length

NFB - Number of Fragment Blocks

MTU - Maximum Transmission Unit
Procedure: IF TL =< MTU THEN Submit this datagram to the next step in datagram processing ELSE IF DF = 1 THEN discard the

datagram ELSE
To produce the first fragment: (1) Copy the original internet header; (2) OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF; (3) NFB <- (MTU-IHL4)/8; (4) Attach the first NFB8 data octets; (5) Correct the header: MF <- 1; TL <- (IHL4)+(NFB8); Recompute Checksum; (6) Submit this fragment to the next step in datagram processing; To produce the second fragment: (7) Selectively copy the internet header (some options are not copied, see option definitions); (8) Append the remaining data; (9) Correct the header: IHL <- (((OIHL*4)-(length of options not copied))+3)/4;
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TL <- OTL - NFB*8 - (OIHL-IHL)*4); FO <- OFO + NFB; MF <- OMF; Recompute Checksum; (10) Submit this fragment to the fragmentation test; DONE.

In the above procedure each fragment (except the last) was made
the maximum allowable size. An alternative might produce less than the maximum size datagrams. For example, one could implement

a fragmentation procedure that repeatly divided large datagrams in

half until the resulting fragments were less than the maximum
transmission unit size.

An Example Reassembly Procedure

For each datagram the buffer identifier is computed as the
concatenation of the source, destination, protocol, and identification fields. If this is a whole datagram (that is both the fragment offset and the more fragments fields are zero), then

any reassembly resources associated with this buffer identifier

are released and the datagram is forwarded to the next step in
datagram processing.

If no other fragment with this buffer identifier is on hand then
reassembly resources are allocated. The reassembly resources consist of a data buffer, a header buffer, a fragment block bit table, a total data length field, and a timer. The data from the

fragment is placed in the data buffer according to its fragment
offset and length, and bits are set in the fragment block bit table corresponding to the fragment blocks received.

If this is the first fragment (that is the fragment offset is
zero) this header is placed in the header buffer. If this is the

last fragment ( that is the more fragments field is zero) the
total data length is computed. If this fragment completes the

datagram (tested by checking the bits set in the fragment block
table), then the datagram is sent to the next step in datagram processing; otherwise the timer is set to the maximum of the

current timer value and the value of the time to live field from
this fragment; and the reassembly routine gives up control. If the timer runs out, the all reassembly resources for this buffer identifier are released. The initial setting of the timer is a lower bound on the reassembly waiting time. This is because

the waiting time will be increased if the Time to Live in the

arriving fragment is greater than the current timer value but will
not be decreased if it is less. The maximum this timer value could reach is the maximum time to live (approximately 4.25 minutes). The current recommendation for the initial timer setting is 15 seconds. This may be changed as experience with
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this protocol accumulates. Note that the choice of this parameter

value is related to the buffer capacity available and the data
rate of the transmission medium; that is, data rate times timer value equals buffer size (e.g., 10Kb/s X 15s = 150Kb). Notation:

FO - Fragment Offset

IHL - Internet Header Length

MF - More Fragments flag

TTL - Time To Live

NFB - Number of Fragment Blocks

TL - Total Length

TDL - Total Data Length

BUFID - Buffer Identifier

RCVBT - Fragment Received Bit Table

TLB - Timer Lower Bound
Procedure: (1) BUFID <- source|destination|protocol|identification; (2) IF FO = 0 AND MF = 0 (3) THEN IF buffer with BUFID is allocated (4) THEN flush all reassembly for this BUFID; (5) Submit datagram to next step; DONE. (6) ELSE IF no buffer with BUFID is allocated (7) THEN allocate reassembly resources with BUFID; TIMER <- TLB; TDL <- 0; (8) put data from fragment into data buffer with BUFID from octet FO8 to octet (TL-(IHL4))+FO8; (9) set RCVBT bits from FO to FO+((TL-(IHL4)+7)/8); (10) IF MF = 0 THEN TDL <- TL-(IHL4)+(FO8) (11) IF FO = 0 THEN put header in header buffer (12) IF TDL # 0 (13) AND all RCVBT bits from 0 to (TDL+7)/8 are set (14) THEN TL <- TDL+(IHL*4) (15) Submit datagram to next step; (16) free all reassembly resources for this BUFID; DONE. (17) TIMER <- MAX(TIMER,TTL); (18) give up until next fragment or timer expires; (19) timer expires: flush all reassembly with this BUFID; DONE.

In the case that two or more fragments contain the same data
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Specification
either identically or through a partial overlap, this procedure

will use the more recently arrived copy in the data buffer and
datagram delivered.

Identification

The choice of the Identifier for a datagram is based on the need to

provide a way to uniquely identify the fragments of a particular
datagram. The protocol module assembling fragments judges fragments to belong to the same datagram if they have the same source, destination, protocol, and Identifier. Thus, the sender must choose the Identifier to be unique for this source, destination pair and

protocol for the time the datagram (or any fragment of it) could be
alive in the internet.

It seems then that a sending protocol module needs to keep a table
of Identifiers, one entry for each destination it has communicated with in the last maximum packet lifetime for the internet. However, since the Identifier field allows 65,536 different values,

some host may be able to simply use unique identifiers independent
of destination.

It is appropriate for some higher level protocols to choose the
identifier. For example, TCP protocol modules may retransmit an identical TCP segment, and the probability for correct reception

would be enhanced if the retransmission carried the same identifier

as the original transmission since fragments of either datagram
could be used to construct a correct TCP segment.

Type of Service
The type of service (TOS) is for internet service quality selection.

The type of service is specified along the abstract parameters
precedence, delay, throughput, and reliability. These abstract

parameters are to be mapped into the actual service parameters of
the particular networks the datagram traverses. Precedence. An independent measure of the importance of this datagram. Delay. Prompt delivery is important for datagrams with this indication. Throughput. High data rate is important for datagrams with this indication.
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Reliability. A higher level of effort to ensure delivery is important for datagrams with this indication. For example, the ARPANET has a priority bit, and a choice between "standard" messages (type 0) and "uncontrolled" messages (type 3), (the choice between single packet and multipacket messages can also be considered a service parameter). The uncontrolled messages tend to be less reliably delivered and suffer less delay. Suppose an internet datagram is to be sent through the ARPANET. Let the internet type of service be given as: Precedence: 5 Delay: 0 Throughput: 1 Reliability: 1 In this example, the mapping of these parameters to those available

for the ARPANET would be to set the ARPANET priority bit on since
the Internet precedence is in the upper half of its range, to select

standard messages since the throughput and reliability requirements
are indicated and delay is not. More details are given on service mappings in "Service Mappings" [8].

Time to Live

The time to live is set by the sender to the maximum time the
datagram is allowed to be in the internet system. If the datagram is in the internet system longer than the time to live, then the datagram must be destroyed.

This field must be decreased at each point that the internet header
is processed to reflect the time spent processing the datagram.

Even if no local information is available on the time actually
spent, the field must be decremented by 1. The time is measured in units of seconds (i.e. the value 1 means one second). Thus, the maximum time to live is 255 seconds or 4.25 minutes. 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.

Some higher level reliable connection protocols are based on

assumptions that old duplicate datagrams will not arrive after a
certain time elapses. The TTL is a way for such protocols to have an assurance that their assumption is met.
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Options The options are optional in each datagram, but required in implementations. That is, the presence or absence of an option is the choice of the sender, but each internet module must be able to parse every option. There can be several options present in the option field. The options might not end on a 32-bit boundary. The internet header must be filled out with octets of zeros. The first of these would be interpreted as the end-of-options option, and the remainder as internet header padding. Every internet module must be able to act on every option. The Security Option is required if classified, restricted, or compartmented traffic is to be passed. Checksum

The internet header checksum is recomputed if the internet header is
changed. For example, a reduction of the time to live, additions or changes to internet options, or due to fragmentation. This checksum

at the internet level is intended to protect the internet header
fields from transmission errors.

There are some applications where a few data bit errors are
acceptable while retransmission delays are not. If the internet

protocol enforced data correctness such applications could not be
supported. Errors Internet protocol errors may be reported via the ICMP messages [3].

3.3. Interfaces

The functional description of user interfaces to the IP is, at best, fictional, since every operating system will have different facilities. Consequently, we must warn readers that different IP implementations may have different user interfaces. However, all IPs

must provide a certain minimum set of services to guarantee that all
IP implementations can support the same protocol hierarchy. This

section specifies the functional interfaces required of all IP
implementations.

Internet protocol interfaces on one side to the local network and on

the other side to either a higher level protocol or an application
program. In the following, the higher level protocol or application
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program (or even a gateway program) will be called the "user" since it is using the internet module. Since internet protocol is a datagram protocol, there is minimal memory or state maintained between datagram transmissions, and each call on the internet protocol module by the

user supplies all information necessary for the IP to perform the
service requested.

An Example Upper Level Interface

The following two example calls satisfy the requirements for the user
to internet protocol module communication ("=>" means returns): SEND (src, dst, prot, TOS, TTL, BufPTR, len, Id, DF, opt => result) where: src = source address dst = destination address prot = protocol TOS = type of service TTL = time to live BufPTR = buffer pointer len = length of buffer Id = Identifier DF = Don’t Fragment opt = option data result = response OK = datagram sent ok Error = error in arguments or local network error

Note that the precedence is included in the TOS and the
security/compartment is passed as an option. RECV (BufPTR, prot, => result, src, dst, TOS, len, opt) where: BufPTR = buffer pointer prot = protocol result = response OK = datagram received ok Error = error in arguments len = length of buffer src = source address dst = destination address TOS = type of service opt = option data
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When the user sends a datagram, it executes the SEND call supplying all the arguments. The internet protocol module, on receiving this call, checks the arguments and prepares and sends the message. If the arguments are good and the datagram is accepted by the local network, the call returns successfully. If either the arguments are bad, or the datagram is not accepted by the local network, the call returns unsuccessfully. On unsuccessful returns, a reasonable report must be made as to the cause of the problem, but the details of such reports are up to individual implementations.

When a datagram arrives at the internet protocol module from the local
network, either there is a pending RECV call from the user addressed or there is not. In the first case, the pending call is satisfied by passing the information from the datagram to the user. In the second case, the user addressed is notified of a pending datagram. If the user addressed does not exist, an ICMP error message is returned to the sender, and the data is discarded.

The notification of a user may be via a pseudo interrupt or similar
mechanism, as appropriate in the particular operating system environment of the implementation. A user’s RECV call may then either be immediately satisfied by a pending datagram, or the call may be pending until a datagram arrives.

The source address is included in the send call in case the sending

host has several addresses (multiple physical connections or logical
addresses). The internet module must check to see that the source address is one of the legal address for this host.

An implementation may also allow or require a call to the internet

module to indicate interest in or reserve exclusive use of a class of
datagrams (e.g., all those with a certain value in the protocol field). This section functionally characterizes a USER/IP interface. The

notation used is similar to most procedure of function calls in high
level languages, but this usage is not meant to rule out trap type service calls (e.g., SVCs, UUOs, EMTs), or any other form of interprocess communication.
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Appendix A: Examples & Scenarios

Example 1: This is an example of the minimal data carrying internet datagram: 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 21 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 123 | Protocol = 1 | header checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+

Example Internet Datagram
Figure 5. Note that each tick mark represents one bit position. This is a internet datagram in version 4 of internet protocol; the internet header consists of five 32 bit words, and the total length of the datagram is 21 octets. This datagram is a complete datagram (not a fragment).
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Example 2: In this example, we show first a moderate size internet datagram (452 data octets), then two internet fragments that might result from the

fragmentation of this datagram if the maximum sized transmission
allowed were 280 octets. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 472 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 123 | Protocol = 6 | header checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Example Internet Datagram
Figure 6.
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Now the first fragment that results from splitting the datagram after
256 data octets. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 276 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=1| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 119 | Protocol = 6 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Example Internet Fragment
Figure 7.
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Internet Protocol
And the second fragment.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 216 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 32 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 119 | Protocol = 6 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Example Internet Fragment
Figure 8.
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Example Internet Datagram
Figure 9.
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Appendix B: Data Transmission Order

The order of transmission of the header and data described in this
document is resolved to the octet level. Whenever a diagram shows a group of octets, the order of transmission of those octets is the normal order in which they are read in English. For example, in the following diagram the octets are transmitted in the order they are numbered. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 | 3 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 6 | 7 | 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | 10 | 11 | 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Transmission Order of Bytes
Figure 10.

Whenever an octet represents a numeric quantity the left most bit in the
diagram is the high order or most significant bit. That is, the bit labeled 0 is the most significant bit. For example, the following diagram represents the value 170 (decimal). 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 0 1 0 1 0 1 0|
+-+-+-+-+-+-+-+-+

Significance of Bits
Figure 11. Similarly, whenever a multi-octet field represents a numeric quantity the left most bit of the whole field is the most significant bit. When

a multi-octet quantity is transmitted the most significant octet is
transmitted first.
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Glossary

1822 BBN Report 1822, "The Specification of the Interconnection of a Host and an IMP". The specification of interface between a host and the ARPANET.

ARPANET leader

The control information on an ARPANET message at the host-IMP
interface.

ARPANET message

The unit of transmission between a host and an IMP in the
ARPANET. The maximum size is about 1012 octets (8096 bits).

ARPANET packet

A unit of transmission used internally in the ARPANET between
IMPs. The maximum size is about 126 octets (1008 bits).

Destination
The destination address, an internet header field. DF Flags The Don’t Fragment bit carried in the flags field. An internet header field carrying various control flags.

Fragment Offset

This internet header field indicates where in the internet
datagram a fragment belongs. GGP header ICMP Gateway to Gateway Protocol, the protocol used primarily

between gateways to control routing and other gateway
functions. Control information at the beginning of a message, segment, datagram, packet or block of data. Internet Control Message Protocol, implemented in the internet module, the ICMP is used from gateways to hosts and between hosts to report errors and make routing suggestions.
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Glossary

Identification

An internet header field carrying the identifying value

assigned by the sender to aid in assembling the fragments of a
datagram. IHL IMP

The internet header field Internet Header Length is the length
of the internet header measured in 32 bit words. The Interface Message Processor, the packet switch of the ARPANET.

Internet Address

A four octet (32 bit) source or destination address consisting
of a Network field and a Local Address field.

internet datagram

The unit of data exchanged between a pair of internet modules
(includes the internet header).

internet fragment

A portion of the data of an internet datagram with an internet
header.

Local Address
The address of a host within a network. The actual mapping of

an internet local address on to the host addresses in a
network is quite general, allowing for many to one mappings. MF module

The More-Fragments Flag carried in the internet header flags
field. An implementation, usually in software, of a protocol or other procedure.

more-fragments flag

A flag indicating whether or not this internet datagram
contains the end of an internet datagram, carried in the internet header Flags field. NFB
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The Number of Fragment Blocks in a the data portion of an
internet fragment. That is, the length of a portion of data measured in 8 octet units.

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Internet Protocol

Glossary

octet Options Padding Protocol Rest Source TCP An eight bit byte. The internet header Options field may contain several options, and each option may be several octets in length.

The internet header Padding field is used to ensure that the
data begins on 32 bit word boundary. The padding is zero. In this document, the next higher level protocol identifier, an internet header field. The local address portion of an Internet Address. The source address, an internet header field. Transmission Control Protocol: A host-to-host protocol for reliable communication in internet environments.

TCP Segment

The unit of data exchanged between TCP modules (including the
TCP header). TFTP Trivial File Transfer Protocol: A simple file transfer protocol built on UDP.

Time to Live

An internet header field which indicates the upper bound on
how long this internet datagram may exist. TOS

Type of Service

Total Length

The internet header field Total Length is the length of the
datagram in octets including internet header and data. TTL

Time to Live
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Glossary

Type of Service

An internet header field which indicates the type (or quality)
of service for this internet datagram. UDP User Version
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User Datagram Protocol: A user level protocol for transaction oriented applications. The user of the internet protocol. This may be a higher level protocol module, an application program, or a gateway program. The Version field indicates the format of the internet header.

September 1981

Internet Protocol

References

[1] Cerf, V., "The Catenet Model for Internetworking," Information
Processing Techniques Office, Defense Advanced Research Projects Agency, IEN 48, July 1978.
[2] Bolt Beranek and Newman, "Specification for the Interconnection of
a Host and an IMP," BBN Technical Report 1822, Revised May 1978.
[3] Postel, J., "Internet Control Message Protocol - DARPA Internet
Program Protocol Specification," RFC 792, USC/Information Sciences Institute, September 1981.
[4] Shoch, J., "Inter-Network Naming, Addressing, and Routing,"
COMPCON, IEEE Computer Society, Fall 1978.
[5] Postel, J., "Address Mappings," RFC 796, USC/Information Sciences
Institute, September 1981.
[6] Shoch, J., "Packet Fragmentation in Inter-Network Protocols,"
Computer Networks, v. 3, n. 1, February 1979.
[7] Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek and
Newman, August 1979.
[8] Postel, J., "Service Mappings," RFC 795, USC/Information Sciences
Institute, September 1981.
[9] Postel, J., "Assigned Numbers," RFC 790, USC/Information Sciences
Institute, September 1981.
[Page 45]

RFC 791 - Internet Protocol

Table of Contents

1. Introduction

2. Overview

3. Specification

Appendix A: Examples & Scenarios

Appendix B: Data Transmission Order

Glossary

References

Preface

1.1. Motivation

1. Introduction

1.2. Scope

1.3. Interfaces

1.4. Operation

2. Overview

2.1. Relation To Other Protocols

2.2. Model Of Operation

2.3. Function Description

2.4. Gateways

3. Specification

3.1. Internet Header Format

3.2. Discussion

3.3. Interfaces

Appendix A: Examples & Scenarios

Appendix B: Data Transmission Order

Glossary

Glossary

Glossary

Glossary

References