Principle and application of TCP/IP protocol Note 25: Internet Protocol (IP) datagram (Datagram)

1. Datagram (Datagram) format:

2. Length Field

(1) Header Length field,Hlen,4 bits

• In 4-byte (or 32bits) units
• Value: 5~15 (that is, the first length is bytes)

(2) Total Length field, sumlength, bits

• In byte units
• Header length (i.e. 4 * hlen) + data length

Note: What is the maximum length of IP grouping?

Answer:65535

3. Service type field

(1) Type of Service (TOS), 8 bits

• The quality of service that the group expects

4. Shard Control Fields

(1) Identification field,identification, bits

• Source Station Each send a group, identity value +1
• (Source IP address, identity)---> All-network unique group identification

(2) Flag field,flags, 3 bits

(3) Slice offset field,fragmentation offset, bits

• In 8 bytes Units

Note: Why do IP datagrams need to be fragmented?

Answer:

Principle and application of TCP/IP protocol Note 25: Internet Protocol (IP) datagram (Datagram)