brian@sdcsvax.UCSD.EDU (Brian Kantor) (08/25/87)
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.
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 must be conservative
in its sending behavior, and liberal in its receiving behavior. That
is, it must be careful to send well-formed datagrams, but must 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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Addressing
To provide for flexibility in assigning address to networks and
allow for the large number of small to intermediate sized networks
the interpretation of the address field is coded to specify a small
number of networks with a large number of host, a moderate number of
networks with a moderate number of hosts, and a large number of
networks with a small number of hosts. In addition there is an
escape code for extended addressing mode.
Address Formats:
High Order Bits Format Class
--------------- ------------------------------- -----
0 7 bits of net, 24 bits of host a
10 14 bits of net, 16 bits of host b
110 21 bits of net, 8 bits of host c
111 escape to extended addressing mode
A value of zero in the network field means this network. This is
only used in certain ICMP messages. The extended addressing mode
is undefined. Both of these features are reserved for future use.
The actual values assigned for network addresses is given in
"Assigned Numbers" [9].
The local address, assigned by the local network, must allow for a
single physical host to act as several distinct internet hosts.
That is, there must be a mapping between internet host addresses and
network/host interfaces that allows several internet addresses to
correspond to one interface. It must 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 "Address
Mappings" [5].
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
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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 (of course, the header is counted in the
total length and not in the fragments).
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.
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.
One example of use of the Don't Fragment feature is to down line
load a small host. A small host could have a boot strap program
that accepts a datagram stores it in memory and then executes it.
The fragmentation and reassembly procedures are most easily
described by examples. The following procedures are example
implementations.
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).
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An Example 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 is still too large.
Notation:
FO - Fragment Offset
IHL - Internet Header Length
DF - Don't Fragment flag
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
Procedure:
IF TL =< MTU THEN Submit this datagram to the next step
in datagram processing ELSE IF DF = 1 THEN discard the
datagram 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;
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TL <- OTL - NFB*8 - (OIHL-IHL)*4);
FO <- OFO + NFB; MF <- OMF; Recompute Checksum;
(10) Submit this fragment to the fragmentation test; DONE.
In the above procedure each fragment (except the last) was made
the maximum allowable size. An alternative might produce less
than the maximum size datagrams. For example, one could implement
a fragmentation procedure that repeatly divided large datagrams in
half until the resulting fragments were less than the maximum
transmission unit size.
An Example 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
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
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Specification
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
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
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Specification
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.
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, delay, throughput, and reliability. 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.
Delay. Prompt delivery is important for datagrams with this
indication.
Throughput. High data rate is important for datagrams with this
indication.
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Reliability. A higher level of effort to ensure delivery is
important for datagrams with this indication.
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:
Precedence: 5
Delay: 0
Throughput: 1
Reliability: 1
In this example, the mapping of these parameters to those available
for the ARPANET would be to set the ARPANET priority bit on since
the Internet precedence is in the upper half of its range, to select
standard messages since the throughput and reliability requirements
are indicated and delay is not. More details are given on service
mappings in "Service Mappings" [8].
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 must be destroyed.
This field must 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 must 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. 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.
Some higher level reliable connection protocols are based on
assumptions that old duplicate datagrams will not arrive after a
certain time elapses. The TTL is a way for such protocols to have
an assurance that their assumption is met.
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Options
The options are optional in each datagram, but required in
implementations. 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
must 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 every option. The
Security Option is required if classified, restricted, 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.
There are some applications where a few data bit errors are
acceptable while retransmission delays are not. If the internet
protocol enforced data correctness such applications could not be
supported.
Errors
Internet protocol errors may be reported via the ICMP messages [3].
3.3. Interfaces
The functional description of user interfaces to the IP is, at best,
fictional, since every operating system will have different
facilities. Consequently, we must warn readers that different IP
implementations may have different user interfaces. However, all IPs
must provide a certain minimum set of services to guarantee that all
IP implementations can support the same protocol hierarchy. This
section specifies the functional interfaces required of all IP
implementations.
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
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Internet Protocol
Specification
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 information necessary for the IP to perform the
service requested.
An Example Upper Level Interface
The following two example calls satisfy the requirements for the user
to internet protocol module communication ("=>" means returns):
SEND (src, dst, prot, TOS, TTL, BufPTR, len, Id, DF, opt => result)
where:
src = source address
dst = destination address
prot = protocol
TOS = type of service
TTL = time to live
BufPTR = buffer pointer
len = length of buffer
Id = Identifier
DF = Don't Fragment
opt = option data
result = response
OK = datagram sent ok
Error = error in arguments or local network error
Note that the precedence is included in the TOS and the
security/compartment is passed as an option.
RECV (BufPTR, prot, => result, src, dst, TOS, len, opt)
where:
BufPTR = buffer pointer
prot = protocol
result = response
OK = datagram received ok
Error = error in arguments
len = length of buffer
src = source address
dst = destination address
TOS = type of service
opt = option data
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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 must 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
user addressed does not exist, an ICMP error message 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.
The source address is included in the send call in case the sending
host has several addresses (multiple physical connections or logical
addresses). The internet module must check to see that the source
address is one of the legal address for this host.
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).
This section functionally characterizes a USER/IP interface. The
notation used is similar to most procedure of function calls in high
level languages, but this usage is not meant to rule out trap type
service calls (e.g., SVCs, UUOs, EMTs), or any other form of
interprocess communication.
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APPENDIX A: 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 5.
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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Internet Protocol
Example 2:
In this example, we show first a moderate size internet datagram (452
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 6.
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Internet Protocol
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 7.
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Internet Protocol
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 8.
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Internet Protocol
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 9.
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Internet Protocol
APPENDIX B: Data Transmission Order
The order of transmission of the header and data described in this
document is resolved to the octet level. Whenever a diagram shows a
group of octets, the order of transmission of those octets is the normal
order in which they are read in English. For example, in the following
diagram the octets are transmitted in the order they are numbered.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 | 3 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 6 | 7 | 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | 10 | 11 | 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Transmission Order of Bytes
Figure 10.
Whenever an octet represents a numeric quantity the left most bit in the
diagram is the high order or most significant bit. That is, the bit
labeled 0 is the most significant bit. For example, the following
diagram represents the value 170 (decimal).
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 0 1 0 1 0 1 0|
+-+-+-+-+-+-+-+-+
Significance of Bits
Figure 11.
Similarly, whenever a multi-octet field represents a numeric quantity
the left most bit of the whole field is the most significant bit. When
a multi-octet quantity is transmitted the most significant octet is
transmitted first.
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September 1981
Internet Protocol
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September 1981
Internet Protocol
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 leader
The control information on an ARPANET message at the host-IMP
interface.
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.
GGP
Gateway to Gateway Protocol, the protocol used primarily
between gateways to control routing and other gateway
functions.
header
Control information at the beginning of a message, segment,
datagram, packet or block of data.
ICMP
Internet Control Message Protocol, implemented in the internet
module, the ICMP is used from gateways to hosts and between
hosts to report errors and make routing suggestions.
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Internet Protocol
Glossary
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.
Internet Address
A four octet (32 bit) source or destination address consisting
of a Network field and a Local Address field.
internet datagram
The unit of data exchanged between a pair of internet modules
(includes the internet header).
internet fragment
A portion of the data of an internet datagram with an internet
header.
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.
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Internet Protocol
Glossary
octet
An eight bit byte.
Options
The internet header Options field may contain several options,
and each option may be several octets in length.
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 local address portion of an Internet Address.
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).
TFTP
Trivial File Transfer Protocol: A simple file transfer
protocol built on UDP.
Time to Live
An internet header field which indicates the upper bound on
how long this internet datagram may exist.
TOS
Type of Service
Total Length
The internet header field Total Length is the length of the
datagram in octets including internet header and data.
TTL
Time to Live
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Internet Protocol
Glossary
Type of Service
An internet header field which indicates the type (or quality)
of service for this internet datagram.
UDP
User Datagram Protocol: A user level protocol for transaction
oriented applications.
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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Internet Protocol
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, Revised May 1978.
[3] Postel, J., "Internet Control Message Protocol - DARPA Internet
Program Protocol Specification," RFC 792, USC/Information Sciences
Institute, September 1981.
[4] Shoch, J., "Inter-Network Naming, Addressing, and Routing,"
COMPCON, IEEE Computer Society, Fall 1978.
[5] Postel, J., "Address Mappings," RFC 796, USC/Information Sciences
Institute, September 1981.
[6] Shoch, J., "Packet Fragmentation in Inter-Network Protocols,"
Computer Networks, v. 3, n. 1, February 1979.
[7] Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek and
Newman, August 1979.
[8] Postel, J., "Service Mappings," RFC 795, USC/Information Sciences
Institute, September 1981.
[9] Postel, J., "Assigned Numbers," RFC 790, USC/Information Sciences
Institute, September 1981.
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