2.1 Relation to Other Protocols ................................... 5
2.2 Model of Operation ............................................ 5
2.3 Function Description .......................................... 7
This document specifies the DoD Standard Internet Protocol. This
document is based on five earlier editions of the ARPA Internet Protocol
Specification, and the present text draws heavily from them. There have
been many contributors to this work both in terms of concepts and in
terms of text. This edition revises the details security,
compartmentation, and precedence features of the internet protocol.
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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Introduction
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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Internet Protocol
Introduction
options include provisions for timestamps, error reports, and special
routing.
The Header Checksum provides a verification that the information used
in processing internet datagram has been transmitted correctly. The
data may contain errors. If the header checksum fails, the internet
datagram is discarded at once by the entity which detects the error.
The internet protocol does not provide a reliable communication
facility. There are no acknowledgments either end-to-end or
hop-by-hop. There is no error control for data, only a header
checksum. There are no retransmissions. There is no flow control.
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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:
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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Overview
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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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 [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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Internet Protocol
Overview
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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Overview
datagram is set to the value of that field in the long datagram plus
NFB.
This procedure can be generalized for an n-way split, rather than
the two-way split described.
To assemble the fragments 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:
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. 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.
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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Internet Protocol
Specification
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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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 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:
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.
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.)
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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Specification
When an internet module routes a datagram it checks to see if
the return route option is present. If it is, it inserts its
own internet address as known in the environment into which this
datagram is being forwarded into the return route at the front
of the address string and increments the length by four.
Not copied on 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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Specification
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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Specification
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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Specification
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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Specification
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:
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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Specification
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:
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.
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):
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
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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Specification
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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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.
