- January 1980
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TABLE OF CONTENTS
PREFACE
1.
INTRODUCTION
1.1
Motivation
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
3.
SPECIFICATION
3.1
Internet Header Format
3.2
Discussion
3.3
Examples & Scenarios
3.4
Interfaces
GLOSSARY
REFERENCES
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PREFACE
- This document specifies the DoD Standard Internet Protocol. This
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- document is based on five earlier editions of the ARPA Internet Protocol
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- Specification, and the present text draws heavily from them. There have
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- been many contributors to this work both in terms of concepts and in
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- terms of text. This edition revises the details security,
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- compartmentation, and precedence features of the internet protocol.
Jon Postel
Editor
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- RFC: 760
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- IEN: 128
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- Replaces: IENs 123, 111,
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- 80, 54, 44, 41, 28, 26
DOD STANDARD
INTERNET PROTOCOL
- 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 promote data reliability, flow control, sequencing, or other
services commonly found in host-to-host protocols.
- 1.3 Interfaces
This protocol is called on by host-to-host protocols in an internet
environment. This protocol calls on local 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 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.
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- 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) may have procedures for making routing decisions and
other functions.
The internet protocol treats each internet datagram
as an independent entity unrelated to any other internet datagram. There are
no connections or logical circuits (virtual or otherwise).
The
internet protocol uses four key mechanisms in providing its service: Type of
Service, Time to Live, Options, and Header Checksum.
The Type of Service
is used to indicate the quality of the service desired; this may be thought of
as selecting among Interactive, Bulk, or Real Time, for example. The type of
service is an abstract or generalized set of parameters which characterize the
service choices provided in the networks that make up the internet. This type
of service indication is to be used by gateways to select the actual
transmission parameters for a particular network, the network to be used for
the next hop, or the next gateway when routing an internet datagram.
The Time to Live is an indication of the lifetime of an internet
datagram. It is set by the sender of the datagram and reduced at the points
along the route where it is processed. If the time to live reaches zero before
the internet datagram reaches its destination, the internet datagram is
destroyed. The time to live can be thought of as a self destruct time limit.
The Options provide for control functions needed or useful in some
situations but unnecessary for the most common communications. The
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options include provisions for
timestamps, error reports, and special routing.
The Header Checksum
provides a verification that the information used in processing internet
datagram has been transmitted correctly. The data may contain errors. If the
header checksum fails, the internet datagram is discarded at once by the
entity which detects the error.
The internet protocol does not provide a
reliable communication facility. There are no acknowledgments either
end-to-end or hop-by-hop. There is no error control for data, only a header
checksum. There are no retransmissions. There is no flow control.
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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 | |Voice| ... | |
+------+ +-----+ +-----+ +-----+
| | | |
+-----+ +-----+ +-----+
| TCP | | RTP | ... | |
+-----+ +-----+ +-----+
| | |
+-------------------------------+
| Internet Protocol |
+-------------------------------+
|
+---------------------------+
| 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.
- 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
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to it. The internet module determines a local network address
for this internet address, in this case it is the address of a gateway. It
sends this datagram and the local network address to the local network
interface.
The local network interface creates a local network header,
and attaches the datagram to 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 should be forwarded to another host in a
second network. The internet module determines a local net address for the
destination host. It calls on the local network interface for that network to
send the datagram.
This local network interface creates a local
network header and attaches the datagram sending the result to the destination
host.
At this destination host the datagram is stripped of the local net
header by the local network interface and handed to the internet module.
The internet module determines that the datagram is for an application
program in this host. It passes the data to the application program in
response to a system 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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- 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 [3]. A name indicates
what we seek. An address indicates where it is. A route indicates how to get
there. The internet protocol deals primarily with addresses. It is the task of
higher level (i.e., host-to-host or application) protocols to make the mapping
from names to addresses. The 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 one octet network number, followed by a three octet local address. This
three octet field is called the "rest" field.
Care must be taken in
mapping internet addresses to local net addresses; a single physical host must
be able to act as if it were several distinct hosts to the extent of using
several distinct internet addresses. A host should also be able to have
several physical interfaces (multi-homing).
That is, a host should be
allowed several physical interfaces to the network with each having several
logical internet addresses.
Examples of address mappings may be found in
reference [4].
Fragmentation
Fragmentation of an internet datagram
may be necessary when it originates in a local net that allows a large packet
size and must
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traverse a local net that limits packets to a smaller size to
reach its destination.
An internet datagram can be marked "don't
fragment." Any internet datagram so marked is not to be internet fragmented
under any circumstances. If internet datagram marked don't fragment cannot be
delivered to its destination without fragmenting it, it is to be discarded
instead.
Fragmentation, transmission and reassembly across a local
network which is invisible to the internet protocol module is called intranet
fragmentation and may be used [5].
The internet fragmentation and
reassembly procedure needs to be able to break a datagram into an almost
arbitrary number of pieces that can be later reassembled. The receiver of the
fragments uses the identification field to ensure that fragments of different
datagrams are not mixed. The fragment offset field tells the receiver the
position of a fragment in the original datagram. The fragment offset and
length determine the portion of the original datagram covered by this
fragment. The more-fragments flag indicates (by being reset) the last
fragment. These fields provide sufficient information to reassemble datagrams.
The identification field is used to distinguish the fragments of one
datagram from those of another. The originating protocol module of an internet
datagram sets the identification field to a value that must be unique for that
source-destination pair and protocol for the time the datagram will be active
in the internet system. The originating protocol module of a complete datagram
sets the more-fragments flag to zero and the fragment offset to zero.
To
fragment a long internet datagram, an internet protocol module (for example,
in a gateway), creates two new internet datagrams and copies the contents of
the internet header fields from the long datagram into both new internet
headers. The data of the long datagram is divided into two portions on a 8
octet (64 bit) boundary (the second portion might not be an integral multiple
of 8 octets, but the first must be). Call the number of 8 octet blocks in the
first portion NFB (for Number of Fragment Blocks). The first portion of the
data is placed in the first new internet datagram, and the total length field
is set to the length of the first datagram. The more-fragments flag is set to
one. The second portion of the data is placed in the second new internet
datagram, and the total length field is set to the length of the second
datagram. The more-fragments flag carries the same value as the long datagram.
The fragment offset field of the second new internet
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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 datagram
that all have the same value for the four fields: identification, source,
destination, and protocol. The combination is done by placing the data portion
of each fragment in the relative position indicated by the fragment offset in
that fragment's internet header. The first fragment will have the fragment
offset zero, and the last fragment will have the more-fragments flag reset to
zero.
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- 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 3.
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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Type of Service: 8 bits
The Type of Service provides
an indication of the abstract parameters of the quality of service desired.
These parameters are to be used to guide the selection of the actual service
parameters when transmitting a datagram through a particular network. Several
networks offer service precedence, which somehow treats high precedence
traffic as more important than other traffic. A few networks offer a Stream
service, whereby one can achieve a smoother service at some cost. Typically
this involves the reservation of resources within the network. Another choice
involves a low-delay vs. high-reliability trade off. Typically networks invoke
more complex (and delay producing) mechanisms as the need for reliability
increases.
Bits 0-2: Precedence. Bit 3: Stream or Datagram.
Bits 4-5: Reliability.
Bit 6: Speed over Reliability.
Bits 7: Speed.
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| | | | | |
| PRECEDENCE | STRM|RELIABILITY| S/R |SPEED|
| | | | | |
+-----+-----+-----+-----+-----+-----+-----+-----+
PRECEDENCE STRM RELIABILITY S/R SPEED
111-Flash Override 1-STREAM 11-highest 1-speed 1-high
110-Flash 0-DTGRM 10-higher 0-rlblt 0-low
11X-Immediate 01-lower
01X-Priority 00-lowest
00X-Routine
The type of service is used to specify the treatment of the datagram
during its transmission through the internet system. In the
discussion (section 3.2) below, a chart shows the relationship of
the internet type of service to the actual service provided on the
ARPANET, the SATNET, and the PRNET.
Total Length: 16 bits
Total Length is the length of the datagram, measured in octets,
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
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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: Don't Fragment This Datagram (DF).
Bit 2: More Fragments Flag (MF).
0 1 2
+---+---+---+
| | D | M |
| 0 | F | F |
+---+---+---+
Fragment Offset: 13 bits
This field indicates where in the datagram this fragment belongs.
The fragment offset is measured in units of 8 octets (64 bits). The
first fragment has offset zero.
Time to Live: 8 bits
This field indicates the maximum time the datagram is allowed to
remain the internet system. If this field contains the value zero,
then the datagram should be destroyed. This field is modified in
internet header processing. The time is measured in units of
seconds. The intention is to cause undeliverable datagrams to be
discarded.
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Protocol: 8 bits
This field indicates the next level
protocol used in the data portion of the internet datagram. The values for
various protocols are specified in reference [6].
Header Checksum: 16
bits
A checksum on the header only. Since some header fields may
change (e.g., time to live), this is recomputed and verified at each point
that the internet header is processed.
The checksum algorithm is:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header. For purposes of computing
the checksum, the value of the checksum field is zero.
This is a simple to
compute checksum and experimental evidence indicates it is adequate, but it is
provisional and may be replaced by a CRC procedure, depending on further
experience.
Source Address: 32 bits
The source address. The
first octet is the Source Network, and the following three octets are the
Source Local Address.
Destination Address: 32 bits
The
destination address. The first octet is the Destination Network, and the
following three octets are the Destination Local Address.
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Options: variable
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 reserved, must be zero
2 bits option class,
5 bits option number.
The option classes are:
0 = control
1 = internet error
2 = experimental debugging and measurement
3 = reserved for future use
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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 4 Security. Used to carry Security, and user
group (TCC) information compatible with DOD
requirements.
0 3 var. Source Routing. Used to route the internet
datagram based on information supplied by the
source.
0 7 var. Return Route. Used to record the route an
internet datagram takes.
0 8 4 Stream ID. Used to carry the stream
identifier.
1 1 var. General Error Report. Used to report errors
in internet datagram processing.
2 4 6 Internet Timestamp.
2 5 6 Satellite 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.
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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.
Security
This option provides a way for DOD hosts to send security and
TCC (closed user groups) parameters through networks whose
transport leader does not contain fields for this information.
The format for this option is as follows:
+--------+--------+---------+--------+
|00000010|00000100|000000SS | TCC |
+--------+--------+---------+--------+
Type=2 Length=4
Security: 2 bits
Specifies one of 4 levels of security
11-top secret
10-secret
01-confidential
00-unclassified
Transmission Control Code: 8 bits
Provides a means to compartmentalize traffic and define
controlled communities of interest among subscribers.
Note that this option does not require processing by the
internet module but does require that this information be passed
to higher level protocol modules. The security and TCC
information might be used to supply class level and compartment
information for transmitting datagrams into or through
AUTODIN II.
Must be copied on fragmentation.
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Source Route
+--------+--------+--------+---------//--------+
|00000011| length | source route |
+--------+--------+--------+---------//--------+
Type=3
The source route option provides a means for the source of an
internet datagram to supply routing information to be used by
the gateways in forwarding the datagram to the destination.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, as well as length-2 octets of source route data.
A source route is composed of a series of internet addresses.
Each internet address is 32 bits or 4 octets. The length
defaults to two, which indicates the source route is empty and
the remaining routing is to be based on the destination address
field.
If the address in destination address field has been reached and
this option's length is not two, the next address in the source
route replaces the address in the destination address field, and
is deleted from the source route and this option's length is
reduced by four. (The Internet Header Length Field must be
changed also.)
Must be copied on fragmentation.
Return Route
+--------+--------+--------+---------//--------+
|00000111| length | return route |
+--------+--------+--------+---------//--------+
Type=7
The return route option provides a means to record the route of
an internet datagram.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, as well as length-2 octets of return route data.
A return route is composed of a series of internet addresses.
The length defaults to two, which indicates the return route is
empty.
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When an internet module routes
a datagram it checks to see if the return route option is present. If it is,
it inserts its own internet address as known in the environment into which
this datagram is being forwarded into the return route at the front of the
address string and increments the length by four.
Not copied on
fragmentation, goes in first fragment only.
Stream Identifier
+--------+--------+---------+--------+
|00001000|00000010| Stream ID |
+--------+--------+---------+--------+
Type=8 Length=4
This option provides a way for the 16-bit SATNET stream
identifier to be carried through networks that do not support
the stream concept.
Must be copied on fragmentation.
General Error Report
+--------+--------+--------+--------+--------+----//----+
|00100001| length |err code| id | |
+--------+--------+--------+--------+--------+----//----+
Type=33
The general error report is used to report an error detected in
processing an internet datagram to the source internet module of
that datagram. The "err code" indicates the type of error
detected, and the "id" is copied from the identification field
of the datagram in error, additional octets of error information
may be present depending on the err code.
If an internet datagram containing the general error report
option is found to be in error or must be discarded, no error
report is sent.
ERR CODE:
0 - Undetermined Error, used when no information is available
about the type of error or the error does not fit any defined
class. Following the id should be as much of the datagram
(starting with the internet header) as fits in the option
space.
1 - Datagram Discarded, used when specific information is
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available about the reason for discarding the datagram can be
reported. Following the id should be the original (4-octets) destination
address, and the (1-octet) reason.
Reason Description
------ -----------
0 No Reason
1 No One Wants It - No higher level protocol or
application program at destination wants this
datagram.
2 Fragmentation Needed & DF - Cannot deliver with out
fragmenting and has don't fragment bit set.
3 Reassembly Problem - Destination could not
reassemble due to missing fragments when time to
live expired.
4 Gateway Congestion - Gateway discarded datagram due
to congestion.
The error report is placed in a datagram with the following
values in the internet header fields:
Version: Same as the datagram in error.
IHL: As computed.
Type of Service: Zero.
Total Length: As computed.
Identification: A new identification is selected.
Flags: Zero.
Fragment Offset: Zero.
Time to Live: Sixty.
Protocol: Same as the datagram in error.
Header Checksum: As computed.
Source Address: Address of the error reporting module.
Destination Address: Source address of the datagram in error.
Options: The General Error Report Option.
Padding: As needed.
Not copied on fragmentation, goes with first fragment.
Internet Timestamp
+--------+--------+--------+--------+--------+--------+
|01000100|00000100| time in milliseconds |
+--------+--------+--------+--------+--------+--------+
Type=68 Length=6
The data of the timestamp is a 32 bit time measured in
milliseconds.
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Not copied on fragmentation,
goes with first fragment
Satellite Timestamp
+--------+--------+--------+--------+--------+--------+
|01000101|00000100| time in milliseconds |
+--------+--------+--------+--------+--------+--------+
Type=69 Length=6
The data of the timestamp is a 32 bit time measured in
milliseconds.
Not copied on fragmentation, goes with first fragment
Padding: variable
The internet header padding is used to ensure that the internet
header ends on a 32 bit boundary. The padding is zero.
- 3.2 Discussion
The implementation of a protocol must be robust. Each implementation
must expect to interoperate with others created by different individuals.
While the goal of this specification is to be explicit about the protocol
there is the possibility of differing
interpretations. In general, an
implementation should be conservative in its sending behavior, and liberal in
its receiving behavior. That is, it should be careful to send well-formed
datagrams, but should accept any datagram that it can interpret (e.g., not
object to technical errors where the meaning is still clear).
The
basic internet service is datagram oriented and provides for the fragmentation
of datagrams at gateways, with reassembly taking place at the destination
internet protocol module in the destination host. Of course, fragmentation and
reassembly of datagrams within a network or by private agreement between the
gateways of a network is also allowed since this is transparent to the
internet protocols and the higher-level protocols. This transparent type of
fragmentation and reassembly is termed "network-dependent" (or intranet)
fragmentation and is not discussed further here.
Internet addresses
distinguish sources and destinations to the host level and provide a protocol
field as well. It is assumed that each protocol will provide for whatever
multiplexing is necessary within a host.
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Addressing
The 8 bit network number, which is the
first octet of the address, has a value as specified in reference [6].
The 24 bit local address, assigned by the local network, should allow
for a single physical host to act as several distinct internet hosts. That is,
there should be mapping between internet host addresses and network/host
interfaces that allows several internet addresses to correspond to one
interface. It should also be allowed for a host to have several physical
interfaces and to treat the datagrams from several of them as if they were all
addressed to a single host. Address mappings between internet addresses and
addresses for ARPANET, SATNET, PRNET, and other networks are described in
reference [4].
Fragmentation and Reassembly.
The internet
identification field (ID) is used together with the source and destination
address, and the protocol fields, to identify datagram fragments for
reassembly.
The More Fragments flag bit (MF) is set if the datagram is
not the last fragment. The Fragment Offset field identifies the fragment
location, relative to the beginning of the original unfragmented datagram.
Fragments are counted in units of 8 octets. The fragmentation strategy is
designed so than an unfragmented datagram has all zero fragmentation information (MF = 0, fragment offset =
0). If an internet datagram is fragmented, its data portion must be
broken on 8 octet boundaries.
This format allows 2**13 = 8192 fragments of 8 octets each for a
total of 65,536 octets. Note that this is consistent with the the
datagram total length field.
When fragmentation occurs, some options are copied, but others
remain with the first fragment only.
Every internet module must be able to forward a datagram of 68
octets without further fragmentation. This is because an internet
header may be up to 60 octets, and the minimum fragment is 8 octets.
Every internet destination must be able to receive a datagram of 576
octets either in one piece or in fragments to be reassembled.
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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.
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).
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 still too large.
Notation:
FO - Fragment Offset
IHL - Internet Header Length
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
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Procedure:
IF TL =< MTU THEN Submit this datagram
to the next step in datagram processing ELSE
To produce the first
fragment:
(1) Copy the original internet header;
(2) OIHL <- IHL;
OTL <- TL; OFO <- FO; OMF <- MF;
(3) NFB <- (MTU-IHL*4)/8;
(4) Attach the first NFB*8 data octets;
(5) Correct the header:
MF
<- 1; TL <- (IHL*4)+(NFB*8);
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; TL <- OTL - NFB*8 - (OIHL-IHL)*4);
FO <- OFO + NFB;
MF <- OMF; Recompute Checksum; (10) Submit this fragment to the
fragmentation test; DONE.
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
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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 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
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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 FO*8 to
octet (TL-(IHL*4))+FO*8;
(9) set RCVBT bits from FO
to FO+((TL-(IHL*4)+7)/8);
(10) IF MF = 0 THEN TDL <- TL-(IHL*4)+(FO*8)
(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
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.
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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,
reliability, and speed. A further concern is the possibility of efficient
handling of streams of datagrams. 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.
Stream or Datagram. Indicates if there will be other
datagrams from this source to this destination at regular frequent intervals
justifying the maintenance of stream processing information.
Reliability.
A measure of the level of effort desired to ensure delivery of this datagram.
Speed over Reliability. Indicates the relative importance of speed and
reliability when a conflict arises in meeting the pair of requests.
Speed. A measure of the importance of prompt delivery of this
datagram.
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:
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Precedence: 5
Stream: 0
Reliability: 1
S/R: 1
Speed: 1
The mapping of these parameters to those available for the ARPANET
would be to set the ARPANET priority bit on since the Internet
priority is in the upper half of its range, to select uncontrolled
messages since the speed and reliability requirements are equal and
speed is preferred.
The following chart presents the recommended mappings from the
internet protocol type of service into the service parameters
actually available on the ARPANET, the PRNET, and the SATNET:
+------------+----------+----------+----------+----------+
|Application | INTERNET | ARPANET | PRNET | SATNET |
+------------+----------+----------+----------+----------+
|TELNET |S/D:stream| T: 3 | R: ptp | T: block |
| on | R:normal| S: S | A: no | D: min |
| TCP |S/R:speed | | | H: inf |
| | S:fast | | | R: no |
+------------+----------+----------+----------+----------+
|FTP |S/D:stream| T: 0 | R: ptp | T: block |
| on | R:normal| S: M | A: no | D: normal|
| TCP |S/R:rlblt | | | H: inf |
| | S:normal| | | R: no |
+------------+----------+----------+----------+----------+
|interactive |S/D:strm* | T: 3 | R: ptp | T: stream|
|narrow band | R:least | S: S | A: no | D: min |
| speech | P:speed | | | H: short |
| | S:asap | | | R: no |
+------------+----------+----------+----------+----------+
|datagram |S/D:dtgrm | T: 3 or 0| R:station| T: block |
| | R:normal| S: S or M| A: no | D: min |
| |S/R:speed | | | H: short |
| | S:fast | | | R: no |
+------------+----------+----------+----------+----------+
key: S/D=strm/dtgrm T=type R=route T=type
R=reliability S=size A=ack D=delay
S/R=speed/rlblt H=holding time
S=speed R=reliability
*=requires stream set up
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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 should be destroyed. This field should 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 should 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.
Options
The options are just that, optional. 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 should 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 the following
options: End of Option List (0), No Operation (1), Source Route (3), Return
Route (7), General Error Report (33), and Internet Timestamp (68). The
Security Option (2) is required only if classified 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.
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- 3.3 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 4.
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 (552 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 5.
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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 6.
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- Internet Protocol Specification
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 7.
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Example 3:
Here, we show an example of a datagram
containing options:
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= 8 |Type of Service| Total Length = 576 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 123 | Protocol = 6 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt. Code = x | Opt. Len.= 3 | option value | Opt. Code = x |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt. Len. = 4 | option value | Opt. Code = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt. Code = y | Opt. Len. = 3 | option value | Opt. Code = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \
\ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example Internet Datagram
Figure 8.
- 3.4 Interfaces
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 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 the necessary
information.
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For example, the following two
calls satisfy the requirements for the user to internet protocol module
communication ("=>" means returns):
SEND (dest, TOS, TTL, BufPTR, len,
Id, DF, options => result)
where:
dest = destination address
TOS = type of service
TTL = time to live
BufPTR = buffer pointer
len = length of buffer
Id = Identifier
DF = Don't Fragment
options = option data
result = response
OK = datagram sent ok
Error = error in arguments or local network error
RECV (BufPTR => result, source, dest, prot, TOS, len)
where:
BufPTR = buffer pointer
result = response
OK = datagram received ok
Error = error in arguments
source = source address
dest = destination address
prot = protocol
TOS = type of service
len = length of buffer
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 should
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
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user addressed does not exist, an error datagram 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.
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).
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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 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
- The Don't Fragment bit carried in the flags field.
- Flags
- An internet header field carrying various control flags.
- Fragment Offset
- This internet header field indicates where in the internet datagram a
fragment belongs.
- header
- Control information at the beginning of a message, segment, datagram,
packet or block of data.
- Identification
- An internet header field carrying the identifying value assigned by the
sender to aid in assembling the fragments of a datagram.
- IHL
- The internet header field Internet Header Length is the length of the
internet header measured in 32 bit words.
- IMP
- The Interface Message Processor, the packet switch of the ARPANET.
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-
- Glossary
- Internet Address
- A four octet (32 bit) source or destination address consisting of a
Network field and a Local Address field.
- internet fragment
- A portion of the data of an internet datagram with an internet header.
- internet datagram
- The unit of data exchanged between a pair of internet modules (includes
the internet header).
- ARPANET leader
- The control information on an ARPANET message at the host-IMP interface.
- 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
- The More-Fragments Flag carried in the internet header flags field.
- module
- 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
- 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.
- octet
- An eight bit byte.
- Options
- The internet header Options field may contain several options, and each
option may be several octets in length. The options are used primarily in
testing situations, for example to carry timestamps.
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- January 1980
- Internet Protocol Glossary
- Padding
- The internet header Padding field is used to ensure that the data begins
on 32 bit word boundary. The padding is zero.
- Protocol
- In this document, the next higher level protocol identifier, an internet
header field.
- Rest
- The 3 octet (24 bit) local address portion of an Internet Address.
- RTP
- Real Time Protocol: A host-to-host protocol for communication of time
critical information.
- Source
- The source address, an internet header field.
- TCP
- 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).
- Total Length
- The internet header field Total Length is the length of the datagram in
octets including internet header and data.
- Type of Service
- An internet header field which indicates the type (or quality) of service
for this internet datagram.
- User
- The user of the internet protocol. This may be a higher level protocol
module, an application program, or a gateway program.
- Version
- The Version field indicates the format of the internet header.
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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, May 1978 (Revised).
[3]
Shoch, J., "Inter-Network Naming, Addressing, and Routing," COMPCON, IEEE
Computer Society, Fall 1978.
[4] Postel, J., "Address Mappings," IEN
115, USC/Information Sciences Institute, August 1979.
[5] Shoch, J.,
"Packet Fragmentation in Inter-Network Protocols," Computer Networks, v. 3, n.
1, February 1979.
[6] Postel, J., "Assigned Numbers," RFC 762, IEN
127, USC/Information Sciences Institute, January 1980.
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