WordPress 2.3 (the blog software I’m using) introduced the concept of canonical URLs, which caused a problem on my host after upgrading. To sum it up, there are many possible ways to access an article or a page in WordPress and the concept of canonical URLs will redirect pages to the permanent link specified in the settings. After upgrading to WordPress >2.3, my blog was no longer reachable, due to an infinite redirection loop:

$ wget blog.ohohlfeld.com
–2008-06-24 18:43:50–  http://blog.ohohlfeld.com/
Resolving blog.ohohlfeld.com… 83.236.4.78
Connecting to blog.ohohlfeld.com|83.236.4.78|:80… connected.
HTTP request sent, awaiting response… 301 Moved Permanently
Location: http://blog.ohohlfeld.com/ [following]
–2008-06-24 18:43:51–  http://blog.ohohlfeld.com/
Reusing existing connection to blog.ohohlfeld.com:80.
HTTP request sent, awaiting response… 301 Moved Permanently
Location: http://blog.ohohlfeld.com/ [following]
–2008-06-24 18:43:51–  http://blog.ohohlfeld.com/
Reusing existing connection to blog.ohohlfeld.com:80.
HTTP request sent, awaiting response… 301 Moved Permanently
Location: http://blog.ohohlfeld.com/ [following]

(….) repeated infinitely

This problem is caused by an incorrectly set host attribute within the HTTP header due to mod_proxy. When using mod_proxy, an Apache server running the proxy will redirect requests to the corresponding web servers. In this internal HTTP request, the Apache proxy sets the Host: to the name of the internal host and thus HTTP_HOST is containing some different host name to what is set by the requesting browser. The original host name is populated in X-Forwarded-Host by mod_proxy. Thus, this is a special issue caused by this non-common Apache configuration.

As WordPress relies on a correctly set HTTP_HOST in some situation (redirects by the URL canonicalization, calling wp-cron, self-tag in ATOM feeds, …), it should be overwritten when a valid X-Forwarded-Host attribute is set in the HTTP header, as Andy pointed out. Basically (without further sanity checks), this can be accomplished by adding the following code to wp-config.php:

if ( isset( $_SERVER['HTTP_X_FORWARDED_HOST'] ) )
 $_SERVER['HTTP_HOST'] = $_SERVER['HTTP_X_FORWARDED_HOST'];

Moreover, when plugins — such as Wassup — rely on aquiring the IP address of the calling user, the REMOTE_ADDRESS can be overwritten, as this will always point to the Apache running mod_proxy. I solved this by adding the following code to wp-config.php:

$_SERVER['REMOTE_ADDR'] = zen_get_ip_address();

The zen_get_ip_address(); procedure will return the correct IP address.

However, as already mentioned, these changes are not nessesarry when some of the discussed HTTP header attribute are not modified by an in-between proxy, which should be the normal case!

Jon Crowcroft published an article about Cold Topics in Networking in ACM SIGCOMM Computer Communication Review issue 1/08. In this article, he gives a rough heuristic on how to classify topics as being cold and gives some examples of cold topics afterwards.

Due to the Internet it is easy to “steal” parts or the complete work of others — e.g. essays, theses or other works assigned to students — and re-use them by not labeling it as the work of others (citing). Writing an essay by using the cut & paste technique to copy text blocks from the Internet is easy and quick. Why should a student spend much time on writing an essay that has been already written before? According to a report by the BBC, Student plagiarism is common in the UK and probably becoming more so. In order to limit plagiarism, universities publish guidelines on how to avoid plagiarism. But what exactly is plagiarism? Wikipedia defines plagiarism as

Plagiarism is the practice of claiming or implying original authorship of (or incorporating material from) someone else’s written or creative work, in whole or in part, into one’s own without adequate acknowledgement.

Can there be something as self-plagiarism? Can we steal something from our own work? Yes, in some sense, and it is a problem in academia. I reported recently, that I’m currently involved in the review process for an academic conference. A couple of days ago, one of the reviewers, who worked on a paper that was also assigned to me, claimed to have found a case of self-plagiarism and notified the conference chairs to check this case. Subsequently, the chairs asked the reviewers to check this claim and re-visit their reviews if needed. In the end, the paper has been rejected due to self-plagiarism.

What happened here and why is it bad to steal from oneself? In a first step, I’m going to redefine the term to steal in context of self-plagiarism. It may be adequate when speaking about plagiarism in the sense of stealing a text, but an author cannot steal his own work. I only used this term to highlight the problem of plagiarism in the introduction of this post. According to Roig, “self-plagiarism occurs when authors reuse their own previously written work or data in a ‘new’ written product without letting the reader know that this material has appeared elsewhere” [Roi06]. Thus, self-plagiarism is more about (deceit and fraudulent) concealment than stealing.

But why can it be a problem in academia when authors are reusing previously written work without citing? Well, it is a problem due to novelty of scientific papers. A research paper should present something now, something that was not know before. A new result, a new algorithm, whatever. This makes it interesting and justifies a new publication. Thus, reusing an existing paper means consciously publishing a known fact by claiming to present something new, e.g. in order to increase one’s Google Scholar rating. Academic conferences want to publish and discuss unpublished work and thus self-plagiarism is a problem. (It is alright to publish an extended version or an article based on several conference papers in an academic journal)

And why is it desirable to do self-plagiarism? Well, reusing a previously published paper is much less work than doing originate research and increases the amount of published papers. The amount of published papers is a simple metric that may be used to guess the “competence” of an researcher (as discussed in an previous post). Thus, the more papers published, the better — publish or perish! This fact may entice an author into doing so.

The workshop is finally over and I’m back to Germany. All in all I have to say that IWQoS was a very interesting workshop, having contributions of a very high quality. I want to present a brief résumé  here, but I’m not giving an extensive review and thus recommend you to take a look at the program on your own.

• Two-state Markov models for describing transmission channels are still popular (e.g. used by Liu et al.)
• Algorithms in the field of Pre-Congestion Notification are subject to performance evaluations, which is a good thing in general as evaluations of RED active queue management have been published when RED was already widely deployed and thus were too late to be taken into account. It seems like this is not the case for PCN.
• An interesting contribution has been made to the field of profile based traffic classification in the work of Hu et al., where data mining techniques are applied to generate distinct behavioral application profiles. The authors present an evaluation of an rule set for BitTorrent and PPLive. In contrast to the techniques presented in our talk about Spam and Traffic Profiling techniques in 2006, this approach seems to be more flexible — at least at first sight.
• YouTube has been again subject to an extensive evaluation. In contrast to the papers presented at the Internet Measurement Conference in 2007, this paper discusses the social networks formed in YouTube and their small world character.
• The invited talk given by a colleague of David Hutchison entitled QoS: (Still) a Grand Challenged? reviewed the evolution of QoS techniques starting from ATM and Broadband ISDN. The conclusion drawn from this talk is that QoS is still a considerable challenge and security and resilience issues need to be taken more seriously, which seems to be reasonable.However, it remains to be seen whether the delivery of 100 MBit/s to the home really changes the world as much as highlighted in the talk. What is known to me about ADSL service providers is that most of the users are not extensively using the big pipe they pay for and rather stick with ocassionally using HTTP and checking their mail. In the first days of ADSL deployment, those access lines were extensively used by power users and thus resulted in a high increase of traffic in the core. However, traffic in the core increases much more slowly with a increasing number of ADSL users nowadays, as most of the users are not using their access link very extensively. I’m wondering if this will be similar for 100 Mbit/s access links in the future.

Goedenavond,

ik ben aan de Universiteit Twente sinds gisteren middag. Het weer is goed en de conferentie interessant. Gisteren was een groot feest met bekende DJ’s.

Well, in order to address my target audience, I better switch from Dutch to English I’m currently at the University of Twente for IWQoS 2008, which is a three day workshop focusing on Quality of Service in telecommunication networks. As the chair mentioned during the opening session, 40 % of the participants are from USA/Canada, 40% from Europe and 20 % from Asia/Australia.

At IWQoS, I will present my regular paper which addresses stochastic packet loss models as used for generating Quality of Experience impairments. This research is motivated by the study of perceptual video quality of video sequences, which are impaired due to transmission failures (packet loss). In this work, we analytically derive the second-order statistics for the amount of packet losses in multiple time scales from finite state Markovian point processes to be used for adapting the model to the packet loss pattern observed in measurements.

The University of Twente has more the style of an American campus than a European; the campus is located outside of the city and contains student housings, a supermarket, restaurants — and can therefore be considered as a city of its own. We probably do not have many similar campuses in Europe and I really like this design. One really feels to be in a university and not just somewhere in a city center, where occasionally some academic facilities are placed.

When I arrived at the hotel yesterday, a huge party (citymoves) was going on at the campus. I guess around 10.000 people must have attended this open air event where several DJs, which are very famous in the Netherlands (Armin van Burren, Marco V, ATB, …), spinned Trance music.

The first day of the conference was quite interesting. I was quite surprised, that several talks addressed the topic of small buffers and buffer sizing in core routers. A talk considered the introduction of small world networks in Bitorrent trackers in order to maximise the clustering coefficient. Although this are good news for the P2P community, service providers might see this as bad news as Bittorrent clients will more likely establish non-regional connections which will cause more traffic on expensive peerings.

A talk presented findings from the analysis of an propriety P2P video streaming system and highlighted the demand for quality of service in such an unreliable multicast network. It was surprising for me to see that 80.000 users were not able to join the stream at all.

When the last session ended at 6 PM, we had a little welcome reception, helping to get to know each other. A good place to meet interesing people. I’m really looking forward to the dinner tomorrow evening.