RFC791: IP: INTERNET PROTOCOL¶
Updated by: 1349, 2474, 6864
September 1981
Information Sciences Institute, University of Southern California
PREFACE¶
This document is based on six earlier editions of the
ARPAInternet Protocol Specification
1. INTRODUCTION¶
Such a system has been called a “catenet”
The internet protocol implements two basic functions:
1. addressing 2. fragmentation
The internet protocol uses four key mechanisms in providing its service:
1. Type of Service,
2. Time to Live,
3. Options,
4. Header Checksum.
2. OVERVIEW¶
2.1. Relation to Other Protocols¶
Protocol Relationships:
+------+ +-----+ +-----+ +-----+
|Telnet| | FTP | | TFTP| ... | ... |
+------+ +-----+ +-----+ +-----+
| | | |
+-----+ +-----+ +-----+
| TCP | | UDP | ... | ... |
+-----+ +-----+ +-----+
| | |
+--------------------------+----+
| Internet Protocol & ICMP |
+--------------------------+----+
|
+---------------------------+
| Local Network Protocol |
+---------------------------+
2.2. Model of Operation¶
Transmission Path:
Application Application
Program Program
\ /
Internet Module Internet Module Internet Module
\ / \ /
LNI-1 LNI-1 LNI-2 LNI-2
\ / \ /
Local Network 1 Local Network 2
2.3. Function Description¶
Addressing:
A `distinction` is made between `names`, `addresses`, and `routes`:
A name indicates what we seek.
An address indicates where it is.
A route indicates how to get there.
Fragmentation:
The identification field is used to distinguish the fragments of one datagram from those of another.
1. identification,
2. source,
3. destination,
4. protocol,
2.4. Gateways¶
Gateways implement internet protocol to forward datagrams between networks.
Gateways also implement the Gateway to Gateway Protocol (GGP) to coordinate routing and other internet control information.
Gateway Protocols:
+-------------------------------+
| Internet Protocol & ICMP & GGP|
+-------------------------------+
| |
+---------------+ +---------------+
| Local Net | | Local Net |
+---------------+ +---------------+
3. SPECIFICATION¶
3.1. Internet Header Format¶
The maximal internet header is 60 octets(60*8bit)
and a typical internet header is 20 octets(20*8bit)
Example Internet Datagram Header:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: 4 bits
indicates the format of the internet header
IHL: 4 bits
Internet Header Length
the length of the internet header in 32 bit words
Type of Service: 8 bits
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
Total Length: 16 bits
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.
But, all hosts must be prepared to accept datagrams of up to 576 octets
hosts only send datagrams larger than 576 octets if they have assurance that
the destination is prepared to accept the larger datagrams.
Identification: 16 bits
An identifying value assigned by the sender to aid in assembling the fragments of a datagram.
Flags: 3 bits
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
indicates where in the datagram this fragment belongs.
is measured in units of 8 octets (64 bits).
The first fragment has offset zero.
Time to Live: 8 bits
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.
Protocol: 8 bits
indicates the next level protocol used in the data portion of the internet datagram.
Header Checksum: 16 bits
A checksum on the header only.
Since some header fields change (e.g., time to live),
this is recomputed and verified at each point that the internet header is processed.
Source Address: 32 bits
The source address.
Destination Address: 32 bits
The destination address.
Options: variable
may appear or not in datagrams.
The option field is variable in length. There may be zero or more options.
detailed below...
Padding: variable
is used to ensure that the internet header ends on a 32 bit boundary.
The padding is zero.
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-type octet is viewed as having 3 fields:
0 1 2 3 4 5 6 7 8
+------+-----+------+-----+-----+-----+-----+-----+
|copied|option class| option number |
+------+-----+------+-----+-----+-----+-----+-----+
1 bit copied flag,
indicates that IF this option is copied into all fragments on fragmentation.
2 bits option class,
00 = control
01 = reserved for future use
10 = debugging and measurement
11 = reserved for future use
5 bits option number.
The following internet options are defined:
CLASS NUMBER LENGTH DESCRIPTION
----- ------ ------ -----------
0 0 - End of Option list.
This option occupies only 1 octet;
it has no length octet.
0 1 - No Operation.
This option occupies only 1 octet;
it has no length octet.
0 2 11 Security.
Used to carry Security, Compartmentation, User Group (TCC),
and Handling Restriction Codes compatible with DOD requirements.
0 3 var. Loose Source Routing.
Used to route the internet datagram based on information supplied by the source.
0 9 var. Strict Source Routing.
Used to route the internet datagram based on information supplied by the source.
0 7 var. Record Route.
Used to trace the route an internet datagram takes.
0 8 4 Stream ID.
Used to carry the stream identifier.
2 4 var. Internet Timestamp.
Specific Option Definitions:
1. End of Option List
+--------+
|00000000|
+--------+
Type=0
This is used at the end of all options
2. No Operation
+--------+
|00000001|
+--------+
Type=1
This option may be used between options
3. Security
+--------+--------+---//---+---//---+---//---+---//---+
|10000010|00001011|SSS SSS|CCC CCC|HHH HHH| TCC |
+--------+--------+---//---+---//---+---//---+---//---+
Type=130 Length=11
a. Security (S field): 16 bits
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)
b. Compartments (C field): 16 bits
An all zero value is used when the information transmitted is not compartmented.
c. Handling Restrictions (H field): 16 bits
d. Transmission Control Code (TCC field): 24 bits
4. Loose Source and Record Route
+--------+--------+--------+---------//--------+
|10000011| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=131
5. Strict Source and Record Route
+--------+--------+--------+---------//--------+
|10001001| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=137
6. Record Route
+--------+--------+--------+---------//--------+
|00000111| length | pointer| route data |
+--------+--------+--------+---------//--------+
Type=7
provides a means to record the route of an internet datagram.
7. Stream Identifier
+--------+--------+--------+--------+
|10001000|00000010| Stream ID |
+--------+--------+--------+--------+
Type=136 Length=4
8. Internet Timestamp
+--------+--------+--------+--------+
|01000100| length | pointer|oflw|flg|
+--------+--------+--------+--------+
| internet address |
+--------+--------+--------+--------+
| timestamp |
+--------+--------+--------+--------+
| . |
.
.
Type = 68
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]
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.
3.2 Discussion¶
Addressing¶
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
Fragmentation and Reassembly¶
The
internet identification field (ID)is used together with thesource addressanddestination address, and theprotocol fields, to identify datagram fragments for reassembly.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.
An Example Fragmentation Procedure¶
General notation:
"=<" means "less than or equal",
"#" means "not equal",
"=" means "equal",
"<-" means "is set to".
"x to y" includes x and excludes y;
for example,
"4 to 7" would include 4, 5, and 6 (but not 7).
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 <- MTU;
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;
TL <- OTL - NFB\*8 - (length of options not copied))+3);
FO <- OFO + NFB; MF <- OMF;
Recompute Checksum;
(10) Submit this fragment to the fragmentation test; DONE.
An Example Reassembly Procedure¶
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;
(17) DONE.
(18) TIMER <- MAX(TIMER,TTL);
(19) give up until next fragment or timer expires;
(20) timer expires: flush all reassembly with this BUFID; DONE.
3.3 Interfaces¶
The functional description of user interfaces to the IP is fictional, since every operating system will have different facilities.
However, all IPs must provide a certain minimum set of services to guarantee that all IP implementations can support the same protocol hierarchy.
An Example Upper Level Interface¶
备注
The following two example calls satisfy the requirements for the user to internet protocol module communication
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
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
APPENDIX A: Examples & Scenarios¶
minimal data carrying internet datagram¶
Example 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 |
+-+-+-+-+-+-+-+-+
moderate size internet datagram (452 data octets)¶
Example 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 = 472 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 123 | Protocol = 6 | header checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \
\ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
备注
two internet fragments that might result from the fragmentation of this datagram if the maximum sized transmission allowed were 280 octets.
first 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 = 276 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=1| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 119 | Protocol = 6 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \
\ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
datagram containing options¶
Example 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= 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
APPENDIX B: Data Transmission Order¶
Transmission Order of Bytes:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Significance of Bits:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 0 1 0 1 0 1 0|
+-+-+-+-+-+-+-+-+
代表: 170 (decimal)
single-octet field: 第0位是最高位
multi-octet field: 第1个octet的第0位是最高位