Posts

The CRC-16-CCITT is a cyclic redundancy check - one among many - defined and derived from polynomial x^16^ + x^12^ + x^5^ + 1. It is used by X.25, HDLC, XMODEM, bluetooth, phy header verification and many others. During some analysis I stumbled across the following implementation: unsigned char ser\_data; static unsigned int crc; crc = (unsigned char)(crc >> 8) | (crc << 8); crc ^= ser\_data; crc ^= (unsigned char)(crc & 0xff) >> 4; crc ^= (crc << 8) << 4; crc ^= ((crc & 0xff) << 4) << 1; The expression @(crc « 8) « 4@ quizzicaled me a little bit because @crc«12@ seems more efficient and is mainly equivalent....

This post recycles the title of a famous paper from “Craig Partridge and Jonathan Stone”:http://www.cc.gatech.edu/classes/AY2002/cs8803d_spring/papers/checksum.pdf from 2000. Traces shows that between 1 packet in 1100 and 1 packet in 32000 fails the TCP checksum - even thought a link layer CRC was used. The TCP/UDP checksum is also ineffective at detecting bus specific errors since these errors with simple summations tend to be self canceling. The paper is worth to read it: it talk about the sources of errors, how TCP react and how to reduce the error rate....

Bluetooth (IEEE 802.15.1) is a clean wireless industry standard using the ISM band (between 2,402 GHz and 2,480 GHz). Version 2.0 + EDR (Enhanced Data Rate) provides a practical data transfer rate up to 2.1 megabits per second. This sounds huge compared to Version 1.0 but practical is can be really awkward if you want to transfer large files. Version 3 of the core specification add a clever extension to increase the data rate: it enables the possibility to use the (hopefully available) WiFi link to transfer data....

In user-space the answer is often simple: use @gettimeofday@ if microseconds resolution is sufficing. Another often used mechanism is to use the time stamp counter - but as mentioned in another blog-post in the lion share of all use cases the advice is often too narrow because of clock variances in SMP/CMP systems and hibernation issues. #define rdtscll(val) \ \_\_asm\_\_ \_\_volatile\_\_("rdtsc" : "=A" (val)) In kernel-space the timing capabilities are a little bit more complex and several functions are provided....

Today I spoted some cacheline misses triggered due to some false sharings in libhashish My CPU model (athlon 64 X2 dual-core) has the following cache and memory structure: instruction cache, data cache, instruction TLB, data TLB, L2 data TLB, L2 instruction TLB and a unified L2 cache, each core (no shared L3-Cache): L1-DTLB: 4K byte pages, fully associative 32 entries (2M/4M pages: 8 entries) L1-ITLB: 4K byte pages, fully associative 32 entries (2M/4M pages: 8 entries) L2 DTLB: 4K byte pages, 4-way associative....

Animated presentation held by sociologist David Harvey and Professor Philip Zimbardo for Royal Society for the encouragement of Arts, Manufactures and Commerce (RSA): “RSA Animate - The Secret Powers of Time”:http://www.youtube.com/watch?v=A3oIiH7BLmg “RSA Animate - Crises of Capitalism”:http://www.youtube.com/watch?v=qOP2V_np2c0

Nothing really interesting these evening - just the usual hacking session. Linus is back from vacation and pent-up patches should be soaked up the next days. Eric submitted one patch after the other and the comment of Davem take the biscuit: “Slow down Eric, you’re on fire :-)”. Mathieu Lacage - let’s call him the ns-3 main developer ;-) - posted a patch where he fixed a uninitialized memory access, spotted by valgrind....

Today I spend some hours to complete a python project. Because the project should expandable and manageable the functionality is splitted into classes, packages and so on - python best practices. But, as long as I program with python I don’t understand why this language is so hyped! Several years back I wanted to leave Perl as the scripting language of choice (sometimes I still use Perl). As far as I remember I looked at Python and Ruby and finally I decided to use Ruby....

Münchner Stadtlauf as a distraction (but only 10 kilometers). PS: the compiler optimization patch is now mainline.

During some off-line discussion with Florian - one of the main developers of TCP SYN cookies - I was a little bit skeptic about the mechanism and the interplay with the TCP window scaling option. First I will describe these two mechanism; later on I will discuss their relationship and interplay. At the end I will discuss the regression and possible solutions. I shifted this off-line discussion to the kernel ml because it is not that trivial as it sounds....