Sent a data packet to xxxhub and found...

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Hello everyone, I am Xuanyuan.

That day, a question suddenly occurred to me:

When I visit GitHub, the website that fascinates thousands of geek programmers, how do the data packets sent from my computer reach the GitHub server on the other side of the ocean? What nodes does it pass through?

Let's explore this question together. Please fasten your seat belts. Xuanyuan's computer network express is about to depart...

IP packets

The Internet connects countless mobile phones, computers, servers, routers, switches and other devices together. If these devices want to communicate through the network, they naturally need a set of communication protocols. TCP/IP is such a set of protocols.

The data sent by these applications, including browsers, are encapsulated layer by layer by HTTP, TCP, and IP protocols, and finally form IP packets, which are sent to the underlying network card.

The IP message is continuously routed and forwarded by nodes in the network and finally reaches the target server.

So how do we know which network nodes the route has passed through during forwarding?

The tracert program on Windows and the traceroute program on Linux can do this.

How do they do it?

IP packets cannot be forwarded indefinitely. What if a loop is set up for forwarding? Wouldn't that be endless? IP packets in the network have a concept of lifetime, which is located in the IP packet header field - TTL: time to live.

Each time it is forwarded, the TTL value will be reduced by 1. If a node finds that the TTL has become 0, it will discard the IP message and send a timeout notification message to the sender of the data message.

Tracert and traceroute make use of this feature of the IP protocol. They increase the TTL value from 1 and observe who sends the notification back to them. This way, they can determine which nodes are passed through during the routing process.

The difference between these two programs is that tracert sends ICMP messages, while traceroute sends UDP messages.

Traceroute

Okay, now that we have covered the basics, let’s try accessing GitHub.

First, I pinged and got the IP address of GitHub: 140.80.121.3. Note that this address may be different for people in different regions.

Next, let’s trace the route:

 F:\ work >tracert 140.82.121.3 
 
 Tracking through up to 30 hops
 Route to lb-140-82-121-3-fra.github.com [140.82.121.3]: 
 
 1 <1 ms <1 ms <1 ms 10.??.??.1
 2 <1 ms <1 ms <1 ms 10.??.??.??
  3 2 ms 1 ms 1 ms 182.150.63.1
 4 * * * Request timed out.
  5 1 ms * 2 ms 171.208.199.81
  6*25ms*202.97.29.45
 7 * * * Request timed out.
  8 36 ms 37 ms 36 ms 202.97.91.190
  9 184 ms 191 ms 185 ms 202.97.27.242
 10 195 ms 194 ms 194 ms xe-10-0-0.mpr4.sjc7.us.zip.zayo.com [64.125.14.45]
 11 190 ms 190 ms 190 ms ae16.cr2.sjc2.us.zip.zayo.com [64.125.31.14]
 12 324 ms 325 ms 324 ms ae27.cs2.sjc2.us.eth.zayo.com [64.125.30.232]
 13 * * 333 ms ae16.cs2.den5.us.zip.zayo.com [64.125.28.215]
 14 334 ms * * ae5.cs4.ord2.us.eth.zayo.com [64.125.29.217]
 15 * 327 ms 325 ms ae3.cs2.lga5.us.eth.zayo.com [64.125.29.212]
 16 * * * Request timed out.
 17 * * * Request timed out.
 18 332 ms 332 ms 340 ms ae0.cs1.lhr15.uk.eth.zayo.com [64.125.29.119]
 19 * * * Request timed out.
 20 343 ms 338 ms * ae4.cs1.ams17.nl.eth.zayo.com [64.125.28.36]
 21 355 ms 353 ms 353 ms ae2.cs1.fra6.de.eth.zayo.com [64.125.29.58]
 22 335 ms 334 ms 338 ms ae1.mcs1.fra6.de.eth.zayo.com [64.125.29.57]
 23 340 ms 341 ms 341 ms 82.98.193.31
 24 * * * Request timed out.
 25 * * * Request timed out.
 26 335 ms 343 ms 343 ms lb-140-82-121-3-fra.github.com [140.82.121.3]

As you can see, after being forwarded by 26 nodes, it finally reaches the GitHub server. In other words, the TTL of the IP message sent by your computer must be at least greater than or equal to 26 to reach GitHub, otherwise it will fail halfway.

Next, let's take a look at where we have been all the way.

1-2 After the data packet is sent from my computer, the first forwarding node it encounters is my local LAN gateway: 10.??.??.1. For security reasons, I have anonymized the IP address and replaced the two middle segments with ?.

After that, the second node is still the LAN address. This shows that the network structure I am in has to go through two levels of LAN routing forwarding before reaching the public network.

3The third forwarding node is a public network address: 182.150.63.1. I checked and found that it is located in Wuhou District, Chengdu, which is consistent with my actual situation.

4 The next fourth routing node is a bit confusing. All three time points are *, and tracert shows that the request has timed out. This means that the tracert program did not receive a notification after setting the TTL to 4, or waited too long. Some nodes in the network may not send a timeout notification for security reasons.

In this way, tracert cannot know who the fourth node is.

5The fifth node is: 171.208.199.81, still in Chengdu.

6The sixth node: 202.97.29.45, arrived in Beijing.

7The seventh node is the same as the fourth and is also invisible.

8The eighth node: 202.97.91.190, arrived in Shanghai.

9The ninth node: 202.97.27.242, still in Shanghai.

10The tenth node: went abroad, California, USA.

We won’t look at the rest, which is the forwarding of various nodes within the United States.

Next, let’s take a look at what kind of path this is.

ChinaNet

After the network data packet leaves our local LAN, it will eventually be connected to a larger backbone network through the metropolitan area network provided by the telecom operator.

There are four main civil backbone networks in mainland China:

• ChinaNet: China Telecom 163 backbone network
• CN2: China Telecom's next generation bearer network
• CHINA169: China Unicom 169 backbone network
• CMNET: China Mobile backbone network

Among them, China Telecom's 163 backbone network and China Unicom's 169 backbone network are the two most important backbone networks, carrying the vast majority of China's Internet traffic.

The network I am on is finally connected to China Telecom's 163 backbone network. Below is a rough network topology diagram of the 163 backbone network.

The 163 backbone network has a total of 9 core nodes across the country:

• Super core: Beijing, Shanghai, Guangzhou
• Ordinary core: Tianjin, Xi'an, Nanjing, Hangzhou, Wuhan, Chengdu

The 9 core nodes are each responsible for a part of mainland China.

International network interconnection equipment (X routers) are also installed under the three super cores of Beijing, Shanghai and Guangzhou. ChinaNet is connected to other operators in the world and exchanges traffic through X routers.

Therefore, if you want to go abroad through the 163 network, you must pass through one of the three core nodes in Beijing, Shanghai and Guangzhou.

GitHub's servers are located in the United States. For a data packet that is about to go abroad, its general journey before leaving the country is as follows:

Local area network -> Municipal network -> Provincial network -> Core node -> International exit -> Overseas access point

This process is consistent with the path traced by tracert above.

You can’t believe it, with just one click, the data packet can actually go to so many places. Computer networks are really a magical thing.

This article is reprinted from the WeChat public account "Programming Technology Universe", which can be followed through the following QR code. To reprint this article, please contact the Programming Technology Universe public account.