Introduction to Internet Protocol (ii)

The previous article analyzed the overall concept of the Internet, from bottom to top, the design of each layer of protocol.

This is from the designer's point of view, today I would like to switch to the user's perspective, to see how users from top to bottom, interacting with these protocols.

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Introduction to Internet Protocol (ii)

Transferred from: http://www.ruanyifeng.com/blog/2012/06/internet_protocol_suite_part_ii.html

Nanyi

(next to the above)

七、一个 Summary

First, make a summary of the previous content.

We already know that network communication is the exchange of data packets. Computer A sends a packet to Computer B, which receives, responds to a packet, and realizes communication between the two computers. The structure of the packet is basically the following:

To send this package, you need to know two addresses:

* Each other's MAC address

* Each other's IP address

With these two addresses, the packet can be sent to the receiver accurately. However, as mentioned earlier, the MAC address has limitations, if the two computers are not on the same subnet, you will not know the other's MAC address, must be forwarded through the gateway.

, computer 1th will send a packet to computer number 4th. It first to determine whether the 4th computer is in the same subnet, the results found not (after the introduction of the method of judgment), so the packet sent to gateway A. Gateway A through the routing protocol, found that 4th computer is located in sub-network B, and the packet sent to Gateway B, Gateway B and then forwarded to computer 4th.

Computer number 1th sends the packet to gateway A, you must know the MAC address of Gateway A. Therefore, the destination address of the packet is actually divided into two situations:

Scene	Packet Address
Same sub-network	Each other's MAC address, the other's IP address
Non-identical sub-network	The MAC address of the gateway, the IP address of the other

Before sending a packet, the computer must determine whether the other person is on the same subnet, and then select the appropriate MAC address. Next, let's see how this process is done in practice.

Eight, the user's Internet settings

8.1 Static IP Address

You bought a new computer, plugged in a network cable, power on, then the computer can surf the Internet?

Usually you have to do some setup. Sometimes, the administrator (or ISP) will tell you the following four parameters, you fill them in the operating system, the computer can connect the Internet:

* The IP address of the machine
* Subnet Mask
* IP address of the gateway
* IP address of DNS

is the Windows System Setup window.

These four parameters are integral and will explain why you need to know them to get online. Because they are given, each time the computer is turned on, it will be assigned the same IP address, so this situation is called "Static IP address Internet".

However, such a setting is professional, the average user is daunting, and if the IP address of a computer remains unchanged, other computers will not be able to use this address, not flexible. For these two reasons, most users use "Dynamic IP address Internet".

8.2 Dynamic IP Address

The so-called "Dynamic IP Address", refers to the computer boot, will automatically assign to an IP address, without human settings. The protocol it uses is called the DHCP protocol.

This protocol stipulates that in each sub-network, one computer is responsible for managing all IP addresses of the network, which is called a "DHCP server". When a new computer joins the network, a "DHCP request" packet must be sent to the "DHCP server" requesting the IP address and the associated network parameters.

As mentioned earlier, if two computers are on the same subnet, you must know the other's MAC address and IP address to send the packet. However, the newly added computer does not know these two addresses, how to send a packet?

The DHCP protocol makes some clever rules.

8.3 DHCP protocol

First, it is an application-layer protocol that is built on top of the UDP protocol, so the entire packet is this:

(1) The first "Ethernet header", set the MAC address of the sender (native) and the MAC address of the receiver (DHCP server). The former is the MAC address of the local network card, the latter do not know, fill in a broadcast address: FF-FF-FF-FF-FF-FF.

(2) Next "IP Header", set the IP address of the sender and the IP address of the receiver. At this time, for both, this machine is not known. The IP address of the issuing party is then set to 0.0.0.0, the IP address of the receiver is set to 255.255.255.255.

(3) The last "UDP header", set the port of the issuing party and the port of the receiver. This section is provided by the DHCP protocol, which is port 68 and the receiver is port 67.

Once this packet is constructed, it can be sent out. Ethernet is broadcast sent, and each computer on the same sub-network receives this packet. Because the receiver's MAC address is ff-ff-ff-ff-ff-ff, do not see who is sent to, so each received this package of the computer, you must also analyze the IP address of the package to determine whether it is sent to their own. When the sender IP address is 0.0.0.0 and the receiver is 255.255.255.255, the DHCP server knows "This package is sent to me" and the other computer can discard the package.

Next, the DHCP server reads out the contents of the packet, assigns the IP address, and sends back a "DHCP response" packet. The structure of this response packet is similar, the MAC address of the Ethernet header is the network card address of both sides, The IP address of the IP header is the IP address of the DHCP server (the issuing party) and the 255.255.255.255 (receiver), the UDP header port is 67 (sender) and 68 (receiver), the IP address assigned to the requester side and the specific parameters of the network are included in the data section.

The newly added computer receives the response packet, so it knows its own IP address, subnet mask, gateway address, DNS server, and so on.

8.4 Internet Settings: summary

In this section, one thing to keep in mind: whether it's a "static IP address" or a "dynamic IP address", the first step in computer surfing is to determine four parameters. These four values are important and worth repeating:

* The IP address of the machine
* Subnet Mask
* IP address of the gateway
* IP address of DNS

With these values, the computer can "surf" the Internet. Next, let's look at an example of how the Internet protocol works when a user accesses a webpage.

九、一个 instances: Accessing Web pages

9.1 Native parameters

We assume that, following the steps in the previous section, the user has set their own network parameters:

* The IP address of the machine: 192.168.1.100
* Subnet Mask: 255.255.255.0
* IP Address of the gateway: 192.168.1.1
* DNS IP address: 8.8.8.8

Then he opens the browser, wants to visit Google, and in the address bar entered the URL: www.google.com.

This means that the browser is sending a Web request packet to Google.

9.2 DNS Protocol

We know that sending a packet must be known to the other's IP address. However, now, we only know the URL www.google.com, do not know its IP address.

The DNS protocol can help us to convert this URL into an IP address. The DNS server is known to be 8.8.8.8, so we send a DNS packet (53 port) to this address.

The DNS server then responds by telling us that Google's IP address is 172.194.72.105. So, we know each other's IP address.

9.3 Subnet Mask

Next, we want to determine whether this IP address is in the same subnet, which will use the subnet mask.

The known subnet mask is 255.255.255.0, the machine uses it to its own IP address 192.168.1.100, do a binary and operation (two digits are 1, the result is 1, otherwise 0), the result is 192.168.1.0; Then Google's IP address 172.194.72 .105 also makes an and operation, which evaluates to 172.194.72.0. These two results are not equal, so the conclusion is that Google is not on the same subnet as the native computer.

Therefore, we want to send a packet to Google, must be forwarded through the gateway 192.168.1.1, that is, the receiver's MAC address will be the gateway's MAC address.

9.4 Application Layer Protocol

The Web page is configured with the HTTP protocol, and the entire packet is constructed like this:

The contents of the HTTP section are similar to the following:

get/http/1.1
Host:www.google.com
Connection:keep-alive
user-agent:mozilla/5.0 (Windows NT 6.1) ...
accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-encoding:gzip,deflate,sdch
accept-language:zh-cn,zh;q=0.8
accept-charset:gbk,utf-8;q=0.7,*;q=0.3
Cookies: ...

We assume that the length of this part is 4960 bytes and it will be embedded in the TCP packet.

9.5 TCP protocol

The TCP packet needs to set the port, the receiver (Google) HTTP port is 80 by default, and the sender (native) port is a randomly generated integer between 1024-65535, assuming 51775.

The header length of the TCP packet is 20 bytes, plus the packet embedded in HTTP, the total length becomes 4980 bytes.

9.6 IP protocol

Then, the TCP packet is then embedded in the IP packet. IP packets need to be set up on both sides of the IP address, which is known, the sender is 192.168.1.100 (native) and the receiver is 172.194.72.105 (Google).

The header length of the IP packet is 20 bytes, plus the embedded TCP packet, the total length becomes 5000 bytes.

9.7 Ethernet Protocol

Finally, the IP packet is embedded in the Ethernet packet. Ethernet packet needs to set the MAC address of both sides, the sender is the local network card MAC address, the receiver is the gateway 192.168.1.1 MAC address (through the ARP protocol).

The data portion of the Ethernet packet, the maximum length is 1500 bytes, and now the IP packet length is 5000 bytes. Therefore, IP packets must be split into four packets. Because each package has its own IP header (20 bytes), the length of the IP packets for the four packets is 1500, 1500, 1500, 560, respectively.

9.8 Server-side response

After the forwarding of multiple gateways, Google's server 172.194.72.105, received the four Ethernet packets.

According to the IP header number, Google put four packages together, take out the full TCP packet, and then read the inside of the "HTTP request", and then make "HTTP response", and then sent back with the TCP protocol.

After the native HTTP response is received, the Web page can be displayed to complete a network communication.

This example ends here, although it has been simplified, but it generally reflects the entire communication process of the Internet Protocol.

Finish

Introduction to Internet Protocol (ii)