Computer network (3)-----IP datagram Format

IP datagram (IP Datagram) format

　　　　

Analytical

(1) The version occupies 4 bits, which is the version of the IP protocol. The IP protocol versions used by both sides of the communication must be consistent. The current widely used IP protocol version number is 4 (that is, IPv4).

(2) The first ministerial degree occupies 4 bits, and the maximum decimal value that can be represented is 15. Note that the unit of the number represented in this field is 32 bits in length (a 32-bit word length is 4 bytes), so when the IP header length is 1111 (that is, the decimal 15), the header length reaches 60 bytes. When the header length of an IP packet is not an integer multiple of 4 bytes, it must be populated with the last fill field. Therefore, the data part will always start at 4-byte integer multiples, which makes it more convenient to implement the IP protocol. The disadvantage of a header length limit of 60 bytes is sometimes not enough. However, this is done in the hope that users will minimize overhead. The most commonly used header length is 20 bytes (that is, the header length is 0101), and no options are used.

(3) Differentiated services accounted for 8, for better service.

(4) Total length length refers to the length of the header and the sum of the data, in bytes. The total Length field is 16 bits, so the maximum length of the datagram is 216-1 = 65535 bytes.

(5) The mark (identification) occupies 16 bits. The IP software maintains a counter in memory, each generating a datagram, the counter adds 1, and assigns this value to the identity field.

(6) Flag is 3-bit, but at present only 2 bits are meaningful.

The lowest bit in the Flag field is recorded as MF (more Fragment). Mf=1 is a datagram that says "There are shards" later. Mf=0 says this is the last of several datagrams.

The one in the middle of the flag field is recorded as DF (Don ' t Fragment), meaning "cannot shard." Shards are allowed only when df=0.

(7) Chip offset is 13 bits. The slice offset indicates the relative position of a piece in the original group after a long grouping in the Shard.

(8) Survival time is 8 bits, and the English abbreviation used for the time-to-live field is TTL, which indicates the lifetime of the datagram in the network. This field is set by the source point at which the datagram is emitted. The aim is to prevent the inability to deliver data in a way that is unrestrained in the Internet and thus consumes network resources in vain. The original design was in seconds as the TTL unit. Each time a router is passed, the TTL is subtracted from the datagram when it is consumed by the router. If the datagram consumes less than 1 seconds on the router, the TTL value is reduced by 1. When the TTL value is 0 o'clock, the datagram is discarded.

(10) First test and accounted for 16 bits. This field only examines the header of the datagram, but does not include the data section. This is because the router has to recalculate the first check and (some fields, such as lifetime, flag, slice offset, and so on) every time a router is passed. Do not test the data section to reduce the amount of computational effort.

(11) The source address occupies 32 bits.

(12) The destination address occupies 32 bits.

(13) Variable part

Computer network (3)-----IP datagram Format