08: Network Layer - IP header - Protocol, Header checksum, SRC & DST addresses, and Options part 7

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IP header

Protocol :

This field also occupies 8 bits. It indicates which protocol in the upper layer  (Transport layer)  the packet will be handed to. To identify which protocol at the Transport layer to take the packet, numbers are utilized to be assigned to each service. The following are the most common protocol numbers.

Header checksum :

This field occupies 16 bits. It it important because it the one that makes sure that IP datagram is intact. When the source sends an IP datagram to the destination, this field is given a value (numbers) that sticks to it until arriving at the receiving machine. When the IP datagram (packet) reaches the receiving machine, it opens the aforementioned field and does some calculations to chek whether the value found is the same as the one existed in the header or not.  If the value is the same, that signifies that the header is intact, but if otherwise, the header is corrupted. Thus, the purpose of this field is ensure the integrity of IP datagram by means of some calculations done at both ends. The following figure clarifies that.

Source address :

This field occupies 32 bits. Clearly, it contains that IP (Internet Protocol) of the source machine, which is mandatory in packet delivery.

Destination address :

This field also occupies 32 bits. Plainly, It encompasses the IP ( Internet Protocol) of the destination machine, which is also obligatory to exit in each packet in order to be sent accross the netwrok.

Options :

This field occupies 32 bits. The normal size of the IP header is 20 bytes, but when options are included, the size increases. It must be a multiple of 32 bits, and to do so, padding which is shown in the very beginning figure, randomly adds some bits so that it becomes an even 32 bit length. Options field contains other subfields as shown below.

It mainly consists of three field : Option Type, Option Length, and Option Data. The first field itself has subfields : Copied (or copied flag), Option Class, and Option Number.

Option Type field occupies 8 bits. It is divided into three subfields, each one has its own function. As for Copied, it is assigned only one bit. If it is set to 0, the option is intended to be copied into all fragments a datagram.  Next, Option Class is allocated 2 bits. It indicates the general category to which the option belongs, 0 is for control options, 2 for debugging and measurement, and 3 means reserved. Last but not least, we have Option Number subfield, it occupies 5 bits. It specifies the kind of option. Option Length indicates the size of the whole option in bytes, without excluding the three subfields. Option Data comprises the data to be transmitted as part of the option.

Now, let us list the most common options that might be included in an IP header.

Option Name	Option Number	Option Class	Option Length (Bytes)	Description-explanation
End Of Options List	0	0	1	It is utilized t mark the end of a list of options, it contains just a single zero byte.
Security	2	0	11	this option is done for the military to refer to the security classification of IP datagrams.
Loose Source Route	3	0	variable	This option signifies that the source of a datagram specifies a list of IP addresses that must be followed in sequence, by having intervening hops in between the devices on the list is allowed.
Strict Source Route	9	0	variable	This option indicates that the transmitted datagram must take a certain route, without any intervention of other routers
Record Route	7	0	variable	This option permits a route taken by a certain datagram to be recorded in datagram itself. Each router the datagram passes through adds its ip address to it,  so that when it arrives at the recipient, th IPs can be extracted.
Timestamp	4	2	variable	This option is similar to the previous one (Record Route). The only difference is that this option, instead of adding  IP addresses to the datagram by each router that handles it,  it adds a timestamp in order that when the datagram reaches the receiver, it will be easy to know how long it takes to arrive.

For more options, see the original source IANA 

This lesson is divided into four parts due to intensive content: first part, second part, third part, fourth part (which is the current one).
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