RFC_791阅读笔记（3部分上）

   关于RFC文档，个人认为还是要看英文版，毕竟很多东西是翻译不过来的，只有读原文才能很好的理解，而实际英语也不是我们想的那么难，只要静下心来，其实很简单！而且读后对你的英语能力提高不是一般的快。

   下边是我的RFC_791（IP）的阅读笔记，暂时还没读完，欢迎大家一起来交流。“//”后的都是自己写的注释

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Internet Protocol

3. SPECIFICATION

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                                                    //version

The Version field indicates the format of the internet header. This
document describes version 4.

IHL: 4 bits                                                    //Internet Header Length in 32 bit word

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                                                    //Type of Service,3 way tradeoff between D,T,R

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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Internet Protocol
Specification

The Network Control precedence designation is intended to be used  //network control is the most important
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 in octets,2^16-1=65535,but normally 576

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  //the reason of choosing 576=64*n+60
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                                             //Identification:used to fragmentation,mapped with one datagram

An identifying value assigned by the sender to aid in assembling the
fragments of a datagram.

Flags: 3 bits                                                      //Flag:sign the first and the last

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                                         //Fragment Offset:the sequence number of fragmentation,in 8 octots

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                                             //Time to Live,the maxinum time of datagram's bounding

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                                             //Protocol:the higher level's protocol

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                                    //Header Checksum:CRC,based on the proper checksum algorithm

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                                    //Source Address

The source address. See section 3.2.

Destination Address: 32 bits                               //Destination Address

The destination address. See section 3.2.

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Internet Protocol
Specification

Options: variable                                             //Option:variable length,more than one

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  //two types:single and option type
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 type:type(1:2:5)+length+data
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                                     //most of the "classes" are 0 
1 = reserved for future use
2 = debugging and measurement
3 = reserved for future use

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The following internet options are defined:           //some example to explain the funtion of the class and number

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

A.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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Specification

B.No Operation                                      //for compartmentation

+--------+
|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.

C.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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Internet Protocol
Specification

Compartments (C field): 16 bits                             //the two parts are designed for DIA

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               //segregate

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.

D.Loose Source and Record Route                        //strict the route indirectly

+--------+--------+--------+---------//--------+
|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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Internet Protocol
Specification

If the address in destination address field has been reached and   //the procession of recording address
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.

E.Strict Source and Record Route                        //strict the route directly

+--------+--------+--------+---------//--------+
|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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Internet Protocol
Specification

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  //why does it remain a constant length?for its' obilgating
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.

F.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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Specification

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.

G.Stream Identifier                            //stream control

+--------+--------+--------+--------+
|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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Specification

H.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.

3.2. Discussion

The implementation of a protocol must be robust. Each implementation  //the implenmentation of protocol:robust,conserative in sender while liberal in receiver
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  //the place of fragmentation(gateways commended) and reassembly(endhost commended)
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  //the address' function:distinguish,protocol,multiplexing
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    //4 types of IP address,two parts
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 //all zero in the network field is used in ICMP
only used in certain ICMP messages. The extended addressing mode  //and the "111" is designed for future use
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  //the mapping between host and its address
single physical host to act as several distinct internet hosts.     //maybe several addresses to one host,or one address to several host 
That is, there must be a mapping between internet host addresses and  //for more exactly description,it's the mapping between interforced and addressed
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      //Address Mapping,the one of most points in Address 
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   //ID,address and protocol are used in fragmentation and reassembly
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  //MF and Fragment Offset field
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 //unfragment-all zero
has all zero fragmentation information (MF = 0, fragment offset =
0). If an internet datagram is fragmented, its data portion must be //fragment-8 octet boundaries
broken on 8 octet boundaries.

This format allows 2**13 = 8192 fragments of 8 octets each for a   //for the fragment offset is 13 bit
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    //the minnum fragment
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                        //DF for embedding 
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                //an example of using DF
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              //some defined notation           
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                 //MTU
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;                                         //NFB is the length of data fragmented in octet
(4) Attach the first NFB*8 data octets;
(5) Correct the header:
MF <- 1; TL <- (IHL*4)+(NFB*8);                                   //must pay attention to the value's unit
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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Specification

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 way of fragmentation is abundance
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   //the buffer identifier
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 //the reassembly resources:
reassembly resources are allocated. The reassembly resources     //data buffer,header buffer,bit table,total data length,timer
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  //must pay attention to the timer
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 lower bound on th reassembly waiting time
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       //the timer is avaible and it's decided by the TTL of the fragments
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;          //the procedure of reassembly
(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   //the case of receiving several same fragments:renvoate
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  //the function of the identification
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  //the choose of identification
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   //the maxnium TTL is 255 seconds
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         //serval options present
option field.

The options might not end on a 32-bit boundary. The internet header    //filled with zeros to make it end on 4 octets boundary
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     //accept a few errors
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].