Wrong MTU symptoms

The first time I  heard about MTU was back in BBS days. And I knew that lowering MTU can be used to fight with collisions on half-duplex networks. But half-duplex are long gone now and usually I'm not aware of current MTU. 

Last time I had to change MTU settings was when I setup VPN server at the university, I worked for. I still vividly remember this problem: PPP interface is up,  I can ping and ssh into the host on remote side, but scp stucks after 1Kb. Luckily for me, there are whole bunch of VPN manuals/faqs and howtos on the net, and almost every one of them advices to lower MTU in case of a problem.

Today I run into another problem, that can be fixed by lowering MTU, though I'm still  not sure what is causing it. My ISP provides me with IPv6 connectivity, as well as IPv4, and some of the features ( like IPTV ) are available only thru v6. So, I configured v6 tunnel on my router, and watched dancing turtle logo  on kame.net and dancing Google on ipv6.google.com. But after that I noticed, that some of the sites I frequent before became unavailable. One of them is doc.python.org.  That was even more weird, because ping6 worked, and it replied to malformed request, but curl -6 http://doc.python.org never returned.

And to my surprise lowering MTU helped.... Does anyone have an idea why? ( I guess that it's due to wrong MTU settings on one of the v6 routers. )

Bibliophilic

Though I'm following BibliOdyssey for almost a year now,  with every new post I'm still amazed by vast variety of subjects people wanted to write books about and how many of them became obsolete with invention of mass production,  (digital) photography and internet. 

From another book oriented blog BookNerd I found out about weird but fascinating online exhibition Making Visible Embryos, which shows how people imagined embryo would look like and how it was used to educate viewers or teach them a moral lesson. 

P.S. Until today I wasn't aware of that "absolete" is rather modern adjective which, according to urbandictionary.com, is defined as  mix of obsolete and absolute.

Debugging dynamic languages

I think CLI debugger is a must-have tool ( though best and simplest debugging tool in existence are debug prints and  asserts ) To my shame I don't have much experience with CLI debuggers for dynamic languages. Moreover, I'm not sure if any debuggers exists for an early scripting languages like sed or shell scripts. The only CLI scripting language debugger I know about is a very convenient python's pdb. 

If anyone knows any CLI debugging tools for languages like Java PHP, Caml or Lisp - please post a link in comments.