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shadows of echoes of memories of songs
j4
j4
Formed with interstitial vacuities
Exhausting karate class last night -- I had a joint lesson with the new girl (a fresher called Vicky who's currently a green belt, going for purple belt in December), and it was hard work keeping up with her as we went through every wasa we'd learnt so far. Then, when we finished and Sensei asked (as he usually does) if we had any questions, she asked about the correct way to do press-ups. Large numbers of press-ups used to be a grading requirement, which is why she was worrying about it, but they're certainly not on the syllabus any more (though obviously they're still a useful thing to do for strength/fitness). Anyway, never one to leave a question unanswered, Sensei gave us (and the guys who'd arrived for their class after ours) another 15 minutes or so on press-ups and sit-ups. Just what I needed. My only consolation was that I managed to rest my dodgy knee and look hard in the process by doing Boys' Press-ups instead of Girls' Press-ups.

Not feeling very confident about the impending grading (December 13th), but then I've not yet felt confident about a grading, and I've not yet failed one. I do need to practise more, though.

I spent the rest of the evening finding out about nameservers, being cryptic about high-speed penguin exchange in email, failing to write job applications, and having a confusing telephone conversation with a random Eastern European woman. This last was because I was trying to get some sense out of HSBC, because I wanted to check my credit card statement online, so I could get the magic number for activating my PayPal account, because ... [trails off into boring house-that-Jack-built anecdotal mooing]. ANYway, I went to www.hsbc.co.uk, entered my Internet Banking number, and it popped up a new secure window... which said something like "Owing to a period of inactivity we have disconnected you from internet banking for your security." I tried reloading the page and so on in case it had cached something, but it still didn't work, and something about their wording made me worry that in fact they'd deactivated my access to Internet Banking altogether (I hadn't used it for months), so I phoned the helldesk. The girl who answered seemed a bit confused, and when I said the magic word "URL" she hastily said "I'll hand you over to our technical people".

So she did, and there followed a period of some confusion while the nice Eastern European lady (I'm going on her accent, okay? Maybe she's actually from Hull, and just talks funny) tried to talk me through the menus in IE for the PC, and I tried to explain to her that I had a Mac but if she just told me what I was looking for then I could probably find it. Anyway, she suggested clearing the cache, which I did, and which had no effect; I then asked if it was likely to be something to do with the proxy, and she started wittering about how she'd need to give me a "proxy exception", and if I was willing to use a standalone PC (I'd already tried, ill-advisedly, to explain our network situation to her by this point) then it'd all work. In the end I just had to say "Look, if you can confirm there's nothing wrong with my Internet Banking account then I can probably fix it at this end". Which I did, FSVO "fix", by bypassing the local proxy.

sion_a later fixed it properly (we hope) by adding ".jsp" and ".jhtml" to the list of Things Not To Cache. And now I know what JHTML is. Which is nice. Well, it's nice to know. JHTML itself seems inherently evil...

It was a good evening for learning things; sion_a gave me a tutorial in the basics of How Networks Work, which was really interesting -- after I said that what I wanted was basically the TCP/IP Colouring Book he pitched it at exactly the right level & explained how our machines talk to each other. :-) He's really very good at explaining technical stuff, although I think sometimes the analogies I use when I'm trying to assimilate information confuse him a bit (probably because they only make sense IN MY HEAD). He then set me a homework assignment to go and find out "how domain names get translated into IP addresses". I'm currently reading RFC 1034 for fun. ("Fun" is a relative concept, okay?)

So anyway. I'm setting myself up for looking extremely stupid here, but: ask me more questions about networking. If I don't know the answers (as I almost certainly won't), I'll go and find out. You see, this stuff makes my brain hurt, and that reassures me that the old grey matter isn't completely dead.

Current Mood: reticulated and decussated

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Comments
From: tamsinj Date: November 5th, 2003 07:29 am (UTC) (Link)
how does a machine know which other machines are on >this< network?

[extra credit for finding the old way to determine this from ip address]
uon From: uon Date: November 5th, 2003 07:37 am (UTC) (Link)
ask me more questions about networking.
  • Describe the sequence of connections in an FTP transaction, and why this makes life difficult for firewalls unless people use "passive FTP".
  • How does traceroute find out which hosts packets pass through?
acronym From: acronym Date: November 5th, 2003 08:00 am (UTC) (Link)
argh argh interstitial vacancies argh vacancy diffusion argh NON-EXISTANT BLOODY ACTIVATED DIFFUSION BARRIERS *thunk*

- A (in PhD mode)
martling From: martling Date: November 5th, 2003 10:28 am (UTC) (Link)
The bit of my brain that skimparses text is about five years behind the rest. I read that as "jargojargojargojargon" and then realised I knew what you were talking about.
acronym From: acronym Date: November 5th, 2003 10:40 am (UTC) (Link)
It was just occasioned by j4's choice of article title :)

- A
martling From: martling Date: November 5th, 2003 10:52 am (UTC) (Link)
Aaaah. Missed that. I suppose your comment was a bit random without that for context, wasn't it...
mobbsy From: mobbsy Date: November 5th, 2003 09:22 am (UTC) (Link)

TCP/IP Colouring Book


That's TCP/IP Illustrated ? :-)

Homework: Pick an arbitrary position on the OSI 7 layer model and argue it loudly. Marks will be awarded for a forthright style and an ability to change your position without admitting you were wrong. Marks will be deducted for lauging at the 9 layer model.

OK, real helpfulness, er..., not sure what sort of thing you're looking at. Routing is probably worth a look for the sysadmin type stuff, or TCP retransmits for the low-level stuff.


martling From: martling Date: November 5th, 2003 09:31 am (UTC) (Link)

  1. What is a routing table and how is it used? (Sion may or may not have covered this earlier)
  2. Why might it become difficult to maintain routing tables manually as a network gets bigger?
  3. How might you solve this problem? (think about it yourself; I'd be interested to hear what you say without looking at existing systems)


(this is fun, by the way (the asking questions that is (though routing's fun too (sort of))))
sion_a From: sion_a Date: November 5th, 2003 10:06 am (UTC) (Link)
What is a routing table and how is it used?

And for extra credit, illustrate with the output of /sbin/route on fire.
imc From: imc Date: November 5th, 2003 03:00 pm (UTC) (Link)
Boys' Press-ups instead of Girls' Press-ups.
So what's the difference (asks a terminally unsporty person)?
and it popped up a new secure window... which said something like "Owing to a period of inactivity we have disconnected you from internet banking for your security."
How very odd. I've never had that problem until after I've successfully logged in and, well, had a period of inactivity.
sion_a later fixed it properly (we hope) by adding ".jsp" and ".jhtml" to the list of Things Not To Cache. And now I know what JHTML is. Which is nice. Well, it's nice to know. JHTML itself seems inherently evil...
I find this rather odd because, apart from anything else, this transaction goes over HTTPS and no cache should ever be caching any HTTPS traffic. Even if the initial form is sent over plain HTTP, I redirect all* my HTTP traffic via one of ntl's Cambridge caches and it's never given me any problems.

As for what JHTML is, I don't even really know myself, although I suspect the J stands for Java, which is inherently evil anyway.

TCP/IP Colouring Book
No, no. . . Coloured Books is the protocol that JaNET used to use for email.


*Er, except the traffic that goes via wwwcache.ox because it's something that is only accessible within the university.
(Deleted comment)
From: tamsinj Date: November 6th, 2003 12:55 am (UTC) (Link)
you forget hubs - why *they're* different, why they're (now) rarely used, but used to be, and why you may want to use one at times ;)
j4 From: j4 Date: November 6th, 2003 02:30 am (UTC) (Link)
Ooh! Ooh! Miss, I already know this one! Well, bits of it. Please excuse any woolly terminology and handwaving.

When a hub receives a packet, it sends it to everything that's connected to it. So it's using up unnecessary bandwidth on all the things that are just going to drop the packet anyway because it's not addressed to them. A switch, on the other hand, maintains a table of the addresses of the things that are connected to it, so it only sends packets where they're meant to go; so even if heaps of packets are being sent to one machine it doesn't clog up all the others.

Sort of. [Waves hands]

(Ask sion_a about me trying to draw this to explain it to myself, if you want a laugh.)

So presumably the bandwidth issue is (at least part of) the "why they're rarely used". Why you might want to use one now... I don't know. I suppose if you wanted to broadcast stuff to everything on your network then a hub might do that better, because you'd only have to send the thing once (and it'd go to everything) rather than sending it to each individual thing. I'm totally guessing now, though.

(See, answering these questions is where I start looking really stupid...)
From: tamsinj Date: November 6th, 2003 03:20 am (UTC) (Link)
*grin* quite correct ;) two machines throwing full-bandwidth data at each other on a hubbed network will kill everything else. [1] on a switched network, they will (on the whole) be able to use the full bandwidth and it'll be just between the two of them. others won't be affected.

....

switches recognise broadcast traffic, and send it to everything - broadcast packets are 'special' (you can find out how/why fairly easily).

put on a network managers hat, and think why you might want all network traffic going to your machine - even if it's not destined there.

[higher end switches can do this for specific ports too]

[1] further directions from here: the 'unusable' level of bandwidth on an ethernet[2] is about 70% of maximum. do you know why?

[2] thinwire (10base2), thickwire (10base5) and hubbed twisted-pair (10baseT)[3] - but we're into history now. potentially useful if you know how to recognise the various types though (run screaming for the first two - specially 2nd)

[3] can you say why it's 10base2/5? once again - arcana
rbarclay From: rbarclay Date: November 6th, 2003 06:22 am (UTC) (Link)
Argue why hubs are the tools of choice when debugging notwork-related application problems.

Let the term "switching hub" go around in your head for a bit. What does such a beast probably do?

Is a hub a repeater?

Why was using 192.0.0.192 a really, really, REALLY stupid idea on HPs part?
(Deleted comment)
imc From: imc Date: November 6th, 2003 04:12 am (UTC) (Link)
I have a hub at home (12 ports, 10Mbit) but it's not in use at the moment because instead I am using a PC with four network cards in it. The reason why I have the hub is because it cost me zero quid (one of Comlab's cast-offs). It will probably be put back into use if I ever get round to networking upstairs, unless I've moved to wireless by then (Comlab isn't giving away any wireless hubs in the near future, though <g>).
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