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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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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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," RFC762, IEN 127, USC/Information
Sciences Institute, January 1980.
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