Network layer protocol description, IP + ARP packet format Detailed principles (MLA 3 prerequisite knowledge network White)

A. We first look at the network layer is a layer of which seven principles of our network infrastructure

is our third layer, the packet inside the IP header, TC header data and the upper
two. Functions of the network layer of
the network layer packet inside the IP address is equivalent to our logic address, a global address in the world, which our computer will be assigned to an address.
Detailed third data packet format.

Version: We now have two versions of IPV4, IPV6, first of all the resources of our IP addresses is limited, we temporarily in the middle of these two, hundreds of millions of people worldwide 60.
Header length: is the length of our head format, does not contain upper layer data
priority and type of service: Decide what data to send, it sends a message to others such as our QQ, while we are still download things, the computer will let you QQ the message sent out first, if not the priority you have to wait for complete download something, you can message.
Total length: header length + length data format, why do we have a minister of the first and overall length, so the computer knows the length of the data to be able to transmit data fragmentation.
We look at the second layer, which is fragmented message, each piece of data transmission, such as a game G 5 can not complete a data packet transmission, data packet transmission requires a plurality of
identifiers: for each segment of the packet to make a symbol
mark: Are also other subsequent segment
period offset: recombination in the destination data
TTL: the life cycle of each packet through a router will reduce the life of one, or a number of data packets will last forever.
Protocol number: description protocol layer data of
the header checksum: to check the integrity of the data packet
source address: your IP address
Destination address: IP address of the other
options: You can enter time data inside the packet, according to the actual situation variable-length
data
four .ARP principle agreement

How do we know each other's computers MAC address, is achieved by ARP, our offer to the data link layer encapsulation

us to understand it, in order to provide the data link layer encapsulation MAC source and destination addresses to it, do not know the data link look at the road layer what my previous blog.
The first step in our host PC1 send data to PC2, check their cache there PC2 MAC address.
The second step, if not we will send an ARP request message PC1 via broadcast mode
the third step, all hosts can receive ARP request message, PC2 have a look at my IP address, reply immediately to unicast, other hosts throw away.
The fourth step, we will save the PC1 PC2 MAC address of the cache, the next time you will definitely use this cache.