rfc760.txt
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TXT
2,014 行
10-secret
01-confidential
00-unclassified
Transmission Control Code: 8 bits
Provides a means to compartmentalize traffic and define
controlled communities of interest among subscribers.
Note that this option does not require processing by the
internet module but does require that this information be passed
to higher level protocol modules. The security and TCC
information might be used to supply class level and compartment
information for transmitting datagrams into or through
AUTODIN II.
Must be copied on fragmentation.
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January 1980
Internet Protocol
Specification
Source Route
+--------+--------+--------+---------//--------+
|00000011| length | source route |
+--------+--------+--------+---------//--------+
Type=3
The source route option provides a means for the source of an
internet datagram to supply routing information to be used by
the gateways in forwarding the datagram to the destination.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, as well as length-2 octets of source route data.
A source route is composed of a series of internet addresses.
Each internet address is 32 bits or 4 octets. The length
defaults to two, which indicates the source route is empty and
the remaining routing is to be based on the destination address
field.
If the address in destination address field has been reached and
this option's length is not two, the next address in the source
route replaces the address in the destination address field, and
is deleted from the source route and this option's length is
reduced by four. (The Internet Header Length Field must be
changed also.)
Must be copied on fragmentation.
Return Route
+--------+--------+--------+---------//--------+
|00000111| length | return route |
+--------+--------+--------+---------//--------+
Type=7
The return route option provides a means to record the route of
an internet datagram.
The option begins with the option type code. The second octet
is the option length which includes the option type code and the
length octet, as well as length-2 octets of return route data.
A return route is composed of a series of internet addresses.
The length defaults to two, which indicates the return route is
empty.
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Internet Protocol
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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Internet Protocol
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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Internet Protocol
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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Internet Protocol
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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Internet Protocol
Specification
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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January 1980
Internet Protocol
Specification
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:
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