Bojie Li

Update (2014-09-29): Due to some inappropriate content in the mirrors configuration file, the configuration file is no longer public, and some links in this article have become dead links, I’m very sorry.

Due to the disk failure of the USTC open source software mirror (mirrors.ustc.edu.cn) disk failure, stephen, tux and I (boj) are not at school, and the mirrors have not fully recovered since the failure in July, it’s time to start over. This time the mirrors rebuild will be completed entirely by students on campus, which is also an opportunity to practice technology. Here, I will briefly explain what parts the open source software mirror includes and how to build it. Since sourceforge is still waiting for us to synchronize, we hope to restore basic services within three days and rebuild the entire system including synchronization within a week.

WTF?

The so-called open source software mirror is to synchronize some GNU/Linux distributions and well-known open source software repositories from the official site. Users can use the software repository mirror nearby by modifying the configuration file to speed up the download and reduce the load on the official site.

Note: The target audience of this article is computer science students and friends who wish to delve deeper into computers.

Today I attended the Windows Azure Camp of Microsoft Student Summer Camp, from approximately 16:00 to 21:30. There were 10 teams, each divided into four groups, among which the Coding group had compulsory and optional questions. The compulsory question was to build a web application on the Windows Azure cloud platform, where users upload pictures, rotate the pictures and display them to the user, and save them to cloud storage. The optional question was to splice the user’s original picture and the rotated picture into one picture. (I didn’t listen to the question, it might not be accurate, please forgive me)

I didn’t work in my own team, but wandered around the venue, chatting with contestants from various teams and seeing what everyone was doing. At first glance, it seemed a bit like the ACM competition.

From 16:00 to 17:30 on August 21, the Poster session of the 2013 MSRA Summer Camp was held in the public exhibition area of Microsoft Research Asia. Participants included Microsoft Scholars attending the summer camp, members of Microsoft clubs from various universities, and MSRA Researchers & Interns.

Originally, I didn’t plan to present anything, but at the request of the event committee, I chose Blog and Freeshell as my presentation projects. Following the committee’s suggestion, I modified it to “Cloud Services in USTC”, a title that seems big and fashionable.

Above picture (click to enlarge): Our Xbox team’s first-place work in today’s activity. The competition was to create the tallest and most beautiful “tower” within 20 minutes using several chopsticks, newspapers, a roll of tape, and a glue stick. The Xbox team scored 100 points for height (the highest) and 70 points for aesthetics, totaling 170 points, winning first place among the 10 teams on site.

At 19:00 on August 23, the “Ice-breaking Journey” of the 0th day of the 2013 Microsoft Student Summer Camp was held on the first floor of Microsoft Building Tower #1. (To complain about the name, “Ice-breaking Journey”, I thought it was a negotiation with the opposing camp) Nearly 200 Microsoft scholars and representatives of Microsoft student clubs from universities across the country participated in the event. As soon as the event started, the host’s phrase “let everyone get to know each other” shocked everyone.

The 802.11a/g wifi router is marked as 54Mbps, divided by 8, at least 6MB/s. Why is the actual speed only over 2M? Most of the explanations on the Internet are vague, so I had to take a stab at it myself, corrections are welcome.

Wifi communication takes place on a given carrier frequency band (channel), just like everyone talking in a hall, everyone’s speech interferes with others.

Why not create more channels? On the one hand, the ISM band that can be used without a license is very limited in the 2.4GHz area suitable for wireless communication; on the other hand, to ensure communication speed, the channel cannot be too narrow. In 802.11a/g (wifi protocol), the channel width is 22MHz, and 14 channels are divided (some of which are not in the ISM band in some countries). Each channel also interferes with the surrounding channels (see the figure below), which is why wireless routers have a dozen channels, but only the distant 1, 6, 11 channels can be used at the same time.

Introduction: This is the second article in the “Entering SIGCOMM 2013” series. For the first time, Google has fully disclosed the design and three-year deployment experience of its data center wide area network (WAN), this paper may be rated as Best Paper. Why does Google use Software Defined Networking (SDN)? How to gradually deploy SDN to existing data centers? Through the paper, we can glimpse a corner of the iceberg of Google’s global data center network.

Huge Waste of Bandwidth

As we all know, network traffic always has peaks and troughs, with peak traffic reaching 23 times the average traffic. To ensure bandwidth demand during peak periods, a large amount of bandwidth and expensive high-end routing equipment must be purchased in advance, while the average usage is only 30%40%. This greatly increases the cost of the data center.

Author’s note: Starting today, this blog will gradually roll out a series of articles, stepping into the upcoming top academic conference in the field of communication, SIGCOMM 2013, to see what Wireless & Networking researchers around the world are doing. Although these seemingly fancy designs may not work at all in a production environment, they at least point us in some possible directions.

Ambient Backscatter: Wireless Communication Out of Thin Air

Absorbing energy from space and using it as a power source, doesn’t that sound crazy? A research team from the University of Washington has created an RFID card that doesn’t need a power source, can draw energy from TV Towers available in every city to support sensors and microcontrollers, and reflect the energy of the TV Tower’s radio signals, enabling autonomous communication between two RFID cards no more than 50~75 cm apart, with a communication speed of up to 1kbps. To borrow an advertising phrase, it’s “We don’t produce signals, we’re just the porters of TV signals“.

The wireless network at Vantone Inn requires username and password authentication. On Windows, as soon as you connect to the “Vantone Inn” access point, a browser window will pop up, displaying the user authentication interface. Visiting any HTTP address will also redirect to the authentication interface. Once authenticated, the wireless network is unobstructed. Today, a classmate asked me, how can this access point be so NB, can force the computer to pop up a window, is it a virus?

I don’t know the answer to this question, so I captured some packets and then Googled it. For ease of understanding, let’s not discuss the automatic pop-up of the browser first, let’s think about how to implement “visiting any HTTP address will redirect to the authentication interface”.

As per the requirements of the Microsoft Scholars Program, I am to provide some suggestions for Bing’s products. I noticed that at MSRA, it seems that most people use Google. Why don’t they use Microsoft’s own Bing? Here are some technical comparisons.

Exact Matching Capability

The search keyword is “b4: experience with a globally-deployed software defined wan”, which is a paper from SIGCOMM 2013, announced around June.

In Google, only three keywords were entered, and the autocomplete has already come out. The first result is the paper itself, the second is the official SIGCOMM website, and all 10 results on the first page are relevant.

USTC mainly has four outbound routes: Education Network (CERNET), Telecom, Mobile, and Unicom.

These four exits each have their own characteristics, let’s go through them one by one:

Education Network

• The full name of the Education Network is “China Education and Research Computer Network” (CERNET), which connects universities and research institutions nationwide. The main node is in Beijing (the headquarters is in Tsinghua Science Park, next to Google), and USTC is the core node in Anhui Province.
• IPv4 bandwidth 2G
• IPv6 bandwidth 2G, relatively loose (Liuwei is IPv6, very fast)
• The north-south interconnection within the Education Network is quite congested during the day, and the network is more smooth at night. The north-south interconnection problem of other operators may be more serious.
• Access to other domestic operators and foreign countries is very slow
• The routing path on the network is not necessarily the shortest geographically, for example, the traffic from USTC to Beijing may have to go through Shanghai.
• Access to IPs outside the “free addresses” defined by CERNET is charged by traffic. Although it does not charge us directly, try to save money for the school :)
• Everyone is advised to use IPv6 when accessing resources within the Education Network