- Version: 4 bits
-
The Version field indicates the format of the internet header. This
document describes version 4.
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IHL: 4 bits
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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
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The Type of Service provides an indication of the abstract
parameters of the quality of service desired. These parameters are
to be used to guide the selection of the actual service parameters
when transmitting a datagram through a particular network. Several
networks offer service precedence, which somehow treats high
precedence traffic as more important than other traffic (generally
by accepting only traffic above a certain precedence at time of high
load). The major choice is a three way tradeoff between low-delay,
high-reliability, and high-throughput.
Bits 0-2: | Precedence.
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Bit 3: | 0 = Normal Delay, 1 = Low Delay.
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Bit 4: | 0 = Normal Throughput, 1 = High Throughput.
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Bit 5: | 0 = Normal Relibility, 1 = High Relibility.
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Bit 6-7: | Reserved for Future Use.
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Precedence
|
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111 - | Network Control | 011 - | Flash
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110 - | Internetwork Control | 010 - | Immediate
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101 - | CRITIC/ECP | 001 - | Priority
|
100 - | Flash Override | 000 - | Routine
|
The use of the Delay, Throughput, and Reliability indications may
increase the cost (in some sense) of the service. In many networks
better performance for one of these parameters is coupled with worse
performance on another. Except for very unusual cases at most two
of these three indications should be set.
The type of service is used to specify the treatment of the datagram
during its transmission through the internet system. Example
mappings of the internet type of service to the actual service
provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET
is given in "Service Mappings" [8].
The Network Control precedence designation is intended to be used
within a network only. The actual use and control of that
designation is up to each network. The Internetwork Control
designation is intended for use by gateway control originators only.
If the actual use of these precedence designations is of concern to
a particular network, it is the responsibility of that network to
control the access to, and use of, those precedence designations.
-
Total Length: 16 bits
-
Total Length is the length of the datagram, measured in octets,
including internet header and data. This field allows the length of
a datagram to be up to 65,535 octets. Such long datagrams are
impractical for most hosts and networks. All hosts must be prepared
to accept datagrams of up to 576 octets (whether they arrive whole
or in fragments). It is recommended that hosts only send datagrams
larger than 576 octets if they have assurance that the destination
is prepared to accept the larger datagrams.
The number 576 is selected to allow a reasonable sized data block to
be transmitted in addition to the required header information. For
example, this size allows a data block of 512 octets plus 64 header
octets to fit in a datagram. The maximal internet header is 60
octets, and a typical internet header is 20 octets, allowing a
margin for headers of higher level protocols.
-
Identification: 16 bits
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An identifying value assigned by the sender to aid in assembling the
fragments of a datagram.
-
Flags: 3 bits
-
Various Control Flags.
Bit 0: | reserved, must be zero
|
Bit 1: | (DF) 0 = May Fragment, 1 = Don't Fragment.
|
Bit 2: | (MF) 0 = Last Fragment, 1 = More Fragments.
|
-
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.
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Time to Live: 8 bits
-
This field indicates the maximum time the datagram is allowed to
remain in the internet system. If this field contains the value
zero, then the datagram must be destroyed. This field is modified
in internet header processing. The time is measured in units of
seconds, but since every module that processes a datagram must
decrease the TTL by at least one even if it process the datagram in
less than a second, the TTL must be thought of only as an upper
bound on the time a datagram may exist. The intention is to cause
undeliverable datagrams to be discarded, and to bound the maximum
datagram lifetime.
-
Protocol: 8 bits
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This field indicates the next level protocol used in the data
portion of the internet datagram. The values for various protocols
are specified in "Assigned Numbers" [9].
-
Header Checksum: 16 bits
-
A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each point
that the internet header is processed.
The checksum algorithm is:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header. For purposes of
computing the checksum, the value of the checksum field is zero.
This is a simple to compute checksum and experimental evidence
indicates it is adequate, but it is provisional and may be replaced
by a CRC procedure, depending on further experience.
-
Source Address: 32 bits
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The source address. See section 3.2.
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Destination Address: 32 bits
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The destination address. See section 3.2.
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Options: variable
-
The options may appear or not in datagrams. They must be
implemented by all IP modules (host and gateways). What is optional
is their transmission in any particular datagram, not their
implementation.
In some environments the security option may be required in all
datagrams.
The option field is variable in length. There may be zero or more
options. There are two cases for the format of an option:
Case 1: A single octet of option-type.
Case 2: An option-type octet, an option-length octet, and the
actual option-data octets.
The option-length octet counts the option-type octet and the
option-length octet as well as the option-data octets.
The option-type octet is viewed as having 3 fields:
1 bit copied flag,
2 bits option class,
5 bits option number.
The copied flag indicates that this option is copied into all
fragments on fragmentation.
0 = not copied
1 = copied
The option classes are:
0 = control
1 = reserved for future use
2 = debugging and measurement
3 = reserved for future use
The following internet options are defined:
CLASS | NUMBER | LENGTH | DESCRIPTION
|
---|
0 | 0 | - | End of Option list.
This option occupies only
1 octet; it has no length octet.
|
0 | 1 | - | No Operation.
This option occupies only 1
octet; it has no length octet.
|
0 | 2 | 11 | Security. Used to carry Security,
Compartmentation, User Group (TCC), and
Handling Restriction Codes compatible with DOD
requirements.
|
0 | 3 | var. | Loose Source Routing. Used to route the
internet datagram based on information
supplied by the source.
|
0 | 9 | var. | Strict Source Routing. Used to route the
internet datagram based on information
supplied by the source.
|
0 | 7 | var. | Record Route. Used to trace the route an
internet datagram takes.
|
0 | 8 | 4 | Stream ID. Used to carry the stream
identifier.
|
2 | 4 | var. | Internet Timestamp.
|
Specific Option Definitions
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Padding: variable
-
The internet header padding is used to ensure that the internet
header ends on a 32 bit boundary. The padding is zero.