IDEA激活码
三、具体内容
3.1 首部格式
对首部内容的一个大概描述如下：
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
Version域指示了首部格式。这份文件描述了Version 4
IHL: 4 bits
首部长度是指用32位字衡量的首部长度，它指向了数据的开始地址。注意对于一个正确的首部来说，最小的长度为5个字。
Type of Service: 8 bits
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
使用Delay，Throughput以及Reliability可能（某种程度上）增加服务的开销。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].
The Network Control precedence designation is intended to be used 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
总长度指以字节为单位的整个数据报的长度，包括internet首部和数据。此字段允许数据报的长度最多为65535个字节。对于大多数主机和网络来说，这样长的数据报是不切实际的。所有主机都必须准备好接受至多576个字节的数据报（无论它们是完整到达还是以片段形式到达）。RFC建议主机只发送大于576个八位字节的数据报，前提是它们能够保证目的地准备接受较大的数据报。
选择数字576是为了在包含必须的首部信息外，可以传输合理大小的数据块。例如，这个大小允许512个字节的数据加上64字节的首部放入一个数据报中。最大的因特网首部是60个字节，常见的因特网首部是20个字节。
Identification: 16 bits
这个值由发送方指派，用于组装分片的datagram。
Flags: 3 bits
两个控制标志位
Bit 0: reserved, must be zero
Bit 1: (DF) 0 = 可以进行分片, 1 = 不可以进行分片.
Bit 2: (MF) 0 = 指代这是分片后的最后一片, 1 = 不是最后一片.
0 1 2
+---+---+---+
| | D | M |
| 0 | F | F |
+---+---+---+
Fragment Offset: 13 bits
这个域指明了在这个datagram中，目前的片的偏移量，单位是字节。第一个片的偏移为0
Time to Live: 8 bits
这个域指示了datagram允许停留在网络中的最长时间。如果值为0，那么datagram需要被丢弃。它的值在Internet header processing时被修改，其度量单位为秒，但是因为每个处理这个datagram的模块都至少需要将它的值减一（即使是在1s内处理完成）。因此TTL被视为一个datagram可以存在的时间的上界。引入这个值的目的是为了剔除那些无法到达终点的datagram，并且限制datagram的最长存活时间。
Protocol: 8 bits
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
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
源地址
Destination Address: 32 bits
目的地址
Options: variable
这个变量可能在数据报中出现，也可能不出现。但是需要在所有IP模块中实现（包含host和gateway）。这个选项的可选是指在传输过程中可以选择这些参数，并不是指在IP模块中是否进行实现。
Padding: variable
这个变量用于对齐，确保数据报的头部长度是32的整数（用0进行对齐）。
参考
