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
This field indicates where in the datagram this fragment belongs.
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September 1981 Internet Protocol Specification
The fragment offset is measured in units of 8 octets (64 bits). The first fragment has offset zero.
Time to Live: 8 bits
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
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
The source address. See section 3.2.
Destination Address: 32 bits
The destination address. See section 3.2.
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September 1981 Internet Protocol Specification
Options: variable
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 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
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