3.1. Internet Header Format
A summary of the contents of the internet header follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example Internet Datagram Header
Figure 4.
Note that each tick mark represents one bit position.
Version: 4 bits
The Version field indicates the format of the internet header. This
document describes version 4.
IHL: 4 bits
Internet Header Length is the length of the internet header in 32
bit words, and thus points to the beginning of the data. Note that
the minimum value for a correct header is 5.
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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 (generally
by accepting only traffic above a certain precedence at time of high
load). The major choice is a three way tradeoff between low-delay,
high-reliability, and high-throughput.
Bits 0-2: Precedence.
Bit 3: 0 = Normal Delay, 1 = Low Delay.
Bits 4: 0 = Normal Throughput, 1 = High Throughput.
Bits 5: 0 = Normal Relibility, 1 = High Relibility.
Bit 6-7: Reserved for Future Use.
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| | | | | | |
| PRECEDENCE | D | T | R | 0 | 0 |
| | | | | | |
+-----+-----+-----+-----+-----+-----+-----+-----+
Precedence
111 - Network Control
110 - Internetwork Control
101 - CRITIC/ECP
100 - Flash Override
011 - Flash
010 - Immediate
001 - Priority
000 - Routine
The use of the Delay, Throughput, and Reliability indications may
increase the cost (in some sense) of the service. In many networks
better performance for one of these parameters is coupled with worse
performance on another. Except for very unusual cases at most two
of these three indications should be set.
The type of service is used to specify the treatment of the datagram
during its transmission through the internet system. Example
mappings of the internet type of service to the actual service
provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET
is given in "Service Mappings" [8].
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The Network Control precedence designation is intended to be used
within a network only. The actual use and control of that
designation is up to each network. The Internetwork Control
designation is intended for use by gateway control originators only.
If the actual use of these precedence designations is of concern to
a particular network, it is the responsibility of that network to
control the access to, and use of, those precedence designations.
Total Length: 16 bits
Total Length is the length of the datagram, measured in octets,
including internet header and data. This field allows the length of
a datagram to be up to 65,535 octets. Such long datagrams are
impractical for most hosts and networks. All hosts must be prepared
to accept datagrams of up to 576 octets (whether they arrive whole
or in fragments). It is recommended that hosts only send datagrams
larger than 576 octets if they have assurance that the destination
is prepared to accept the larger datagrams.
The number 576 is selected to allow a reasonable sized data block to
be transmitted in addition to the required header information. For
example, this size allows a data block of 512 octets plus 64 header
octets to fit in a datagram. The maximal internet header is 60
octets, and a typical internet header is 20 octets, allowing a
margin for headers of higher level protocols.
Identification: 16 bits
An identifying value assigned by the sender to aid in assembling the
fragments of a datagram.
Flags: 3 bits
Various Control Flags.
Bit 0: reserved, must be zero
Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment.
Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.
0 1 2
+---+---+---+
| | D | M |
| 0 | F | F |
+---+---+---+
Fragment Offset: 13 bits
This field indicates where in the datagram this fragment belongs.
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The fragment offset is measured in units of 8 octets (64 bits). The
first fragment has offset zero.
Time to Live: 8 bits
This field indicates the maximum time the datagram is allowed to
remain in the internet system. If this field contains the value
zero, then the datagram must be destroyed. This field is modified
in internet header processing. The time is measured in units of
seconds, but since every module that processes a datagram must
decrease the TTL by at least one even if it process the datagram in
less than a second, the TTL must be thought of only as an upper
bound on the time a datagram may exist. The intention is to cause
undeliverable datagrams to be discarded, and to bound the maximum
datagram lifetime.
Protocol: 8 bits
This field indicates the next level protocol used in the data
portion of the internet datagram. The values for various protocols
are specified in "Assigned Numbers" [9].
Header Checksum: 16 bits
A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each point
that the internet header is processed.
The checksum algorithm is:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header. For purposes of
computing the checksum, the value of the checksum field is zero.
This is a simple to compute checksum and experimental evidence
indicates it is adequate, but it is provisional and may be replaced
by a CRC procedure, depending on further experience.
Source Address: 32 bits
The source address. See section 3.2.
Destination Address: 32 bits
The destination address. See section 3.2.
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Options: variable
The options may appear or not in datagrams. They must be
implemented by all IP modules (host and gateways). What is optional
is their transmission in any particular datagram, not their
implementation.
In some environments the security option may be required in all
datagrams.
The option field is variable in length. There may be zero or more
options. There are two cases for the format of an option:
Case 1: A single octet of option-type.
Case 2: An option-type octet, an option-length octet, and the
actual option-data octets.
The option-length octet counts the option-type octet and the
option-length octet as well as the option-data octets.
The option-type octet is viewed as having 3 fields:
1 bit copied flag,
2 bits option class,
5 bits option number.
The copied flag indicates that this option is copied into all
fragments on fragmentation.
0 = not copied
1 = copied
The option classes are:
0 = control
1 = reserved for future use
2 = debugging and measurement
3 = reserved for future use
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The following internet options are defined:
CLASS NUMBER LENGTH DESCRIPTION
----- ------ ------ -----------
0 0 - End of Option list. This option occupies only
1 octet; it has no length octet.
0 1 - No Operation. This option occupies only 1
octet; it has no length octet.
0 2 11 Security. Used to carry Security,
Compartmentation, User Group (TCC), and
Handling Restriction Codes compatible with DOD
requirements.
0 3 var. Loose Source Routing. Used to route the
internet datagram based on information
supplied by the source.
0 9 var. Strict Source Routing. Used to route the
internet datagram based on information
supplied by the source.
0 7 var. Record Route. Used to trace the route an
internet datagram takes.
0 8 4 Stream ID. Used to carry the stream
identifier.
2 4 var. Internet Timestamp.
Specific Option Definitions
End of Option List
+--------+
|00000000|
+--------+
Type=0
This option indicates the end of the option list. This might
not coincide with the end of the internet header according to
the internet header length. This is used at the end of all
options, not the end of each option, and need only be used if
the end of the options would not otherwise coincide with the end
of the internet header.
May be copied, introduced, or deleted on fragmentation, or for
any other reason.
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No Operation
+--------+
|00000001|
+--------+
Type=1
This option may be used between options, for example, to align
the beginning of a subsequent option on a 32 bit boundary.
May be copied, introduced, or deleted on fragmentation, or for
any other reason.
Security
This option provides a way for hosts to send security,
compartmentation, handling restrictions, and TCC (closed user
group) parameters. The format for this option is as follows:
+--------+--------+---//---+---//---+---//---+---//---+
|10000010|00001011|SSS SSS|CCC CCC|HHH HHH| TCC |
+--------+--------+---//---+---//---+---//---+---//---+
Type=130 Length=11
Security (S field): 16 bits
Specifies one of 16 levels of security (eight of which are
reserved for future use).
00000000 00000000 - Unclassified
11110001 00110101 - Confidential
01111000 10011010 - EFTO
10111100 01001101 - MMMM
01011110 00100110 - PROG
10101111 00010011 - Restricted
11010111 10001000 - Secret
01101011 11000101 - Top Secret
00110101 11100010 - (Reserved for future use)
10011010 11110001 - (Reserved for future use)
01001101 01111000 - (Reserved for future use)
00100100 10111101 - (Reserved for future use)
00010011 01011110 - (Reserved for future use)
10001001 10101111 - (Reserved for future use)
11000100 11010110 - (Reserved for future use)
11100010 01101011 - (Reserved for future use)
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Compartments (C field): 16 bits
An all zero value is used when the information transmitted is
not compartmented. Other values for the compartments field
may be obtained from the Defense Intelligence Agency.
Handling Restrictions (H field): 16 bits
The values for the control and release markings are
alphanumeric digraphs and are defined in the Defense
Intelligence Agency Manual DIAM 65-19, "Standard Security
Markings".
Transmission Control Code (TCC field): 24 bits
Provides a means to segregate traffic and define controlled
communities of interest among subscribers. The TCC values are
trigraphs, and are available from HQ DCA Code 530.
Must be copied on fragmentation. This option appears at most
once in a datagram.
Loose Source and Record Route
+--------+--------+--------+---------//--------+
|10000011| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=131
The loose source and record route (LSRR) option provides a means
for the source of an internet datagram to supply routing
information to be used by the gateways in forwarding the
datagram to the destination, and to record the route
information.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, the pointer octet, and length-3 octets of route
data. The third octet is the pointer into the route data
indicating the octet which begins the next source address to be
processed. The pointer is relative to this option, and the
smallest legal value for the pointer is 4.
A route data is composed of a series of internet addresses.
Each internet address is 32 bits or 4 octets. If the pointer is
greater than the length, the source route is empty (and the
recorded route full) and the routing is to be based on the
destination address field.
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If the address in destination address field has been reached and
the pointer is not greater than the length, the next address in
the source route replaces the address in the destination address
field, and the recorded route address replaces the source
address just used, and pointer is increased by four.
The recorded route address is the internet module's own internet
address as known in the environment into which this datagram is
being forwarded.
This procedure of replacing the source route with the recorded
route (though it is in the reverse of the order it must be in to
be used as a source route) means the option (and the IP header
as a whole) remains a constant length as the datagram progresses
through the internet.
This option is a loose source route because the gateway or host
IP is allowed to use any route of any number of other
intermediate gateways to reach the next address in the route.
Must be copied on fragmentation. Appears at most once in a
datagram.
Strict Source and Record Route
+--------+--------+--------+---------//--------+
|10001001| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=137
The strict source and record route (SSRR) option provides a
means for the source of an internet datagram to supply routing
information to be used by the gateways in forwarding the
datagram to the destination, and to record the route
information.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, the pointer octet, and length-3 octets of route
data. The third octet is the pointer into the route data
indicating the octet which begins the next source address to be
processed. The pointer is relative to this option, and the
smallest legal value for the pointer is 4.
A route data is composed of a series of internet addresses.
Each internet address is 32 bits or 4 octets. If the pointer is
greater than the length, the source route is empty (and the
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recorded route full) and the routing is to be based on the
destination address field.
If the address in destination address field has been reached and
the pointer is not greater than the length, the next address in
the source route replaces the address in the destination address
field, and the recorded route address replaces the source
address just used, and pointer is increased by four.
The recorded route address is the internet module's own internet
address as known in the environment into which this datagram is
being forwarded.
This procedure of replacing the source route with the recorded
route (though it is in the reverse of the order it must be in to
be used as a source route) means the option (and the IP header
as a whole) remains a constant length as the datagram progresses
through the internet.
This option is a strict source route because the gateway or host
IP must send the datagram directly to the next address in the
source route through only the directly connected network
indicated in the next address to reach the next gateway or host
specified in the route.
Must be copied on fragmentation. Appears at most once in a
datagram.
Record Route
+--------+--------+--------+---------//--------+
|00000111| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=7
The record route option provides a means to record the route of
an internet datagram.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, the pointer octet, and length-3 octets of route
data. The third octet is the pointer into the route data
indicating the octet which begins the next area to store a route
address. The pointer is relative to this option, and the
smallest legal value for the pointer is 4.
A recorded route is composed of a series of internet addresses.
Each internet address is 32 bits or 4 octets. If the pointer is
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greater than the length, the recorded route data area is full.
The originating host must compose this option with a large
enough route data area to hold all the address expected. The
size of the option does not change due to adding addresses. The
intitial contents of the route data area must be zero.
When an internet module routes a datagram it checks to see if
the record route option is present. If it is, it inserts its
own internet address as known in the environment into which this
datagram is being forwarded into the recorded route begining at
the octet indicated by the pointer, and increments the pointer
by four.
If the route data area is already full (the pointer exceeds the
length) the datagram is forwarded without inserting the address
into the recorded route. If there is some room but not enough
room for a full address to be inserted, the original datagram is
considered to be in error and is discarded. In either case an
ICMP parameter problem message may be sent to the source
host [3].
Not copied on fragmentation, goes in first fragment only.
Appears at most once in a datagram.
Stream Identifier
+--------+--------+--------+--------+
|10001000|00000010| Stream ID |
+--------+--------+--------+--------+
Type=136 Length=4
This option provides a way for the 16-bit SATNET stream
identifier to be carried through networks that do not support
the stream concept.
Must be copied on fragmentation. Appears at most once in a
datagram.
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Internet Timestamp
+--------+--------+--------+--------+
|01000100| length | pointer|oflw|flg|
+--------+--------+--------+--------+
| internet address |
+--------+--------+--------+--------+
| timestamp |
+--------+--------+--------+--------+
| . |
.
.
Type = 68
The Option Length is the number of octets in the option counting
the type, length, pointer, and overflow/flag octets (maximum
length 40).
The Pointer is the number of octets from the beginning of this
option to the end of timestamps plus one (i.e., it points to the
octet beginning the space for next timestamp). The smallest
legal value is 5. The timestamp area is full when the pointer
is greater than the length.
The Overflow (oflw) [4 bits] is the number of IP modules that
cannot register timestamps due to lack of space.
The Flag (flg) [4 bits] values are
0 -- time stamps only, stored in consecutive 32-bit words,
1 -- each timestamp is preceded with internet address of the
registering entity,
3 -- the internet address fields are prespecified. An IP
module only registers its timestamp if it matches its own
address with the next specified internet address.
The Timestamp is a right-justified, 32-bit timestamp in
milliseconds since midnight UT. If the time is not available in
milliseconds or cannot be provided with respect to midnight UT
then any time may be inserted as a timestamp provided the high
order bit of the timestamp field is set to one to indicate the
use of a non-standard value.
The originating host must compose this option with a large
enough timestamp data area to hold all the timestamp information
expected. The size of the option does not change due to adding
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timestamps. The intitial contents of the timestamp data area
must be zero or internet address/zero pairs.
If the timestamp data area is already full (the pointer exceeds
the length) the datagram is forwarded without inserting the
timestamp, but the overflow count is incremented by one.
If there is some room but not enough room for a full timestamp
to be inserted, or the overflow count itself overflows, the
original datagram is considered to be in error and is discarded.
In either case an ICMP parameter problem message may be sent to
the source host [3].
The timestamp option is not copied upon fragmentation. It is
carried in the first fragment. Appears at most once in a
datagram.
Padding: variable
The internet header padding is used to ensure that the internet
header ends on a 32 bit boundary. The padding is zero.
