Rack Mount Servers - Build them or Buy them?

Question:
This is the canonical question for "Should I build coomputing hardware myself?" questions.

I have put together countless PCs, but never a large server. The geek in me says build it, but the realist in me says let the manufacturer handle it when there is a problem. Ignoring the time penalty involved with the initial assembly time of a built one, which is a better solution? Have you ever run into a problem with a home build server that would have been solved easier/quicker/cheaper by going with a manufacturer? Are there any features that manufacturers give that aren't easily attainable with a home built server?
Answer: Buy them.  And buy them from alternative sources if you need to be frugal - Craigslist, Ebay, Dell Outlet, etc.
If you end up building them - go with SuperMicro - great gear.
But Commercial Servers will have better out of bandwidth management, better systems management, better support, etc.
And if you need to pinch pennies - use third party memory (i.e. Crucial) - its cheaper and just as good.
Comment: http://www.abmx.com for the win! (see http://www.abmx.com/low-cost-1u-server)  They have *cheep cheep* low-end servers built with Supermicro innards, for less than a user's workstation.  We've bought several for non-critical functions and they just run and run and run...
Comment: ;I ntegrated one step driver update for everything including upgrading the firmware on the drives.
Comment: that's why I asked about home or work - at home if I had a rack (and I do) - I would try out a homebrew server (SuperMicro) but with the recent economic condition and alot of people doing upgrade (which is counterintuitive to the economic condition) - craigslist and ebay have had a lot of decent servers on them and its alot less work to just drive to a parking lot, hand over $100 and come home with a nice 1U or 2U server.
Comment: Exactly what are the management features you've seen in a Dell/HP/IBM server that you cannot get from a DIY server with, say, Supermicro/Tyan mobos, Intel NICs and say Adaptec/LSI controllers?
Answer: Buy.

Warranty.
Engineering Design - heat, airflow
Parts all guranteed to work together

We bought a server from a local white box store.  worst server we ever had.  Built exactly to our specs, but the RAID card didn't like the Motherboard.   Ended up costing us more than the price of the server in strange errors, testing, rebuilding, ordering other cards, and more testing.
Comment: Heat is a very big thing. You want to pay extra for the engineering that solves this.
Comment: If you buy a Supermicro or Tyan chassis, you have the same type of "design/engineering" that Dell/IBM or HP have in terms of heat management.
Answer: Buy buy buy.  A server is not a PC and the enthusiast/hobbyist in you is leading you down the path of wrongfulness.  When you have real live users accessing them and relying on them for their daily work, solidity and reliability are crucial factors.
Answer: Go with Dell.  Warranty is no issue when you need it.  Get it for the length of time you want and the service level you need.  Just had a drive fail in a storage server and we had it in 2.5 hours on a Saturday.  With a custom bult unit there is often an issue over what has failed and who is responsible. You really want to avoid the finger pointing.
Also have had great servcie and suoppot on IBM and HP but Dell usually beats on initial price.
Answer: Buy.
The best locations I've seen so far have have been auctions from companies that have folded. I was recently asked to find a new server  to match the cost and specs of a previous server purchase.
Turns out that the previous server was bought for a couple of thousand when originally it cost over 50 thousand. GBP here, but I think the difference is clear. Even the normal second-hand routes couldn't come close to the price of a sell-off after a company folds.
Answer: If you've had the pleasure of following the StackOverflow server build stories (podcast and blog entries), the lesson sounds like:

you should really only consider BUY. There's a caveat to that though: you must value your time.

Just a simple problem with a ServeRAID controller not liking Western Digital SAS drives, but preferring the Hitachi brand. Never mind issues like: mobo/RAID card compatibility, mobo/NIC, power supplies, etc. I couldn't imagine trying to assemble a server together yourself with various parts, and having no warranty on the entire box as a single unit, onsite vendor service, etc.
Having the internals assembled and tested for compatibility are a critical advantage for the big brands.
Answer: Definitely, BUY them.
The cost saving might look a bit tempting if you try to build it by yourself, but when you encounter a disaster scenario (and believe me, you ARE going to encounter that), you're going to save yourself LOTS of time.
Pros:

Better design (airflow, cabling)
Easier to look for a driver, software updates, bios, firmware updates (all from the vendor place)
Better remote management support (you can reboot and see the booting screen remotely)
WARRANTY

Cons:

Price
Answer: It is perfectly possible to custom build a server to rival anything on the market but you won't do so by buying cheap components at bargain prices. As the price difference for equivalent machines from a manufacturer or built from parts will be quite small, with the latter normally being more expensive, there needs to be a really good reason to go down that path.
When you buy a brand name server, and I'm not talking about some weird brand nobody has ever heard of before, you're (usually) getting a system that will have been assembled with solid components that are have been tested to work together. It will also be backed up be a decent warranty.
If you build your own you may well run into all sorts of issues caused by some of the components not playing nicely with some of the others. You will have no warranty on the complete unit, only warranty on some of the components. You can certainly forget about calling the manufacturer and having them on-site, with a whole set of components, within a few hours to repair the server with minimal downtime.
Comment: Or 3PM Sunday afternoon when the server in question runs the accounting software and the auditors will be there 8AM the following morning. :(
Comment: +1 Last paragraph really speaks to this.  Great server can be built but a great wararnty is also rather good.  Especially at 2:00AM on a weekend.
Answer: Do you care about the system or will it be mission critical? Buy.
Is it easily replaceable and/or really simple (web server, compute node, DNS server)? Build from a SuperMicro barebone chassis.
Is your time worthless and so is your data? Build from all different parts.
I work in an academic setting so I often end up building (or spec'ing a beige box) to save money. It works but it's not great. SuperMicro barebones take the pain out of it mostly because the motherboard is actually good and for most servers you don't need other peripherals. They like 3ware RAID cards as well. All my compute nodes are dual Xeon 5300 2U chassis from SuperMicro... But if one failed tomorrow, I wouldn't be in a bind.
My core file servers are Dell and HP machines with 24x7 support contracts. That way I can get parts quickly.
My last experiences with a full custom spec'd beige box was a disaster... Vendor sold us a support contract and his only spare motherboard on hand was out of province (1 day delay) and DOA (5 days delay to get replacement)... The big problem with building is getting spare parts quickly. A sys admin I know had to wait 5 weeks for a replacement SATA backplane from Chembro a few years ago, 6TB of storage was offline until they got that part. Not Good.
Answer: Don't build a server yourself if you intend to rely on it for anything important -- If you're comparing parts apples-to-apples as it were the price differences are typically not huge, and the ability to call one vendor and get warranty service/support is almost worth the price markup the first time something breaks and you have to run around trying to get it fixed.
If you are put off by the price of Dell's hardware you may want to consider SuperMicro or other "whitebox" vendors - roughly equivalent quality, good warranty coverage and typically slightly lower price points.
Answer: As many people before me have answered, I'm going to go with buy. How much is your time worth? Sure you may save $200-300 up front on a server by building it yourself. How much do you think your time is worth to the company on an hourly basis? Even if it's really low, say $30 an hour, $300 comes out to 10 hours. I can pretty much guarantee you that a self-built server is going to suck out way more than 10 hours of your life during the time you maintain it.
Working at a startup where a previous admin skimped on gear, I've had no end of headaches dealing with generic 4U rackmount servers built out of home PC hardware. Luckily we only have a few left now since we came to the conclusion long ago that it's cheaper to scrap them and buy something engineered and supported than waste our time playing hobbyist PC shop.
Answer: smaller than 3U - buy.
larger than 2U - build
Answer: Building can be worth the risk at the very low end, 2 - 4 servers colo'd with "this guy I know", and at the really high end where you are running all custom code in all custom datacenters (google, top-tier hpc) or just have cheap nocmonkey labor laying around.   Nearly everywhere in between its professional malpractice to build.  There's more reasons than I could probably fit in the character limit of this post, but they really all boil down to this: are you in the business of competing on the best price/performance/compatibility tradeoffs of a ~dozen part x86 server system, or are you in some other business?  Spend your time on  your actual business.
Answer: Building a good compact 1U or 2U server is not as easy as drawing up a decent tower model. There aren't a dozen good sites testing all the components, cooling more a challange, parts more expensive and harder to come by. I have always bought for work, and built for home.
But if I would install a rack at home (I won't. Too much noise), I would build those servers as well since I am a maker at heart and just love DIY when it comes to computing gear.
Answer: I build or very small websites or development ( SuperMicro ) and for clients I buy Dell 1u and 2u servers.
Answer: I suppose it depends on the volume and reliability you're looking for. Mom & Pop shop with 5u worth of machines? Might as well make them to save money.
If you want reliability, then you want homogeny, and you need to buy your servers. I'd recommend against buying from non-primary vendors, unless it's some place like CDW. I buy my network equipment refurbished, not my servers.
Answer: Definitely buy. And if you're going to get many servers, buy them from the same manufacturer. You won't have to deal with different diagnostic procedures and different technical support if you need it one day.
In the past (long time ago) I was used to build my own servers (mainly because it seemed cheaper) but I can tell you that you won't be able to buy separate parts and reach the same quality of design than servers from professionals.
This said, you now have a large choice of brands : Dell is one of my favorites because their web site is very useful to compare configurations.
Answer: Have you considered Amazon's EC2 for this? Might be easier/cheaper than having a server in house. Also, it allows for easy replication and creation of new hardware with scalers in front.
Answer: Buy but try to stay away from refurb.  I have built a few servers in my day and even though everything should work well together there will be something just doesn't.  Also as others have mentioned you get the engineering, warrenty, and everything working together.  Also if it is important to you the out of band management is well worth the cost.
Our development servers are all Dell or IBM refurbs that are coming upon the end of their life (7+ years old) but purchased three years ago.  I can no longer find replacement parts for many of the parts.
Comment: why can't you just buy more old servers of the same type - ebay for parts :)
Comment: I can't waste money in our budget buying more old parts that are doomed to fail sooner rather than later.  Lately I have been forced to build due to budget constraints.  Hoping the bite in the ass that I know is coming comes MUCH later.
Answer: I'd say build can be worthwhile in the following circumstances:

minimum 2U case, I'm more comfortable with 3U or bigger
server that you want to be able to quickly rebuild, but can live with it being down for a while
you either have some spares or are willing to buy them.  You need spare power supply, motherboard, disk controller, disks, ram.

The one time I did this for an important production server, it was just a storage server - a very simple windows configuration, just a server stuffed full of disks on a good raid card.  It was cheaper than a comparable HP server, but not a huge amount.  At the time we like the flexibility of knowing that we could get it back up and running quickly if we had any problem.
Answer: For our production servers we went with Dell, I knew I would have someone onsite in 4 hours if something happened to the hardware.  Servers have better fans, airflow and included software for monitoring.  Tech support is able to see what you purchased, knows every aspect of the system and how each piece interactes with each other.  We'd also never consider buying second hand for production, you never know how they hard drives were abused.
For pre-production & the dev labs, we went with custom built, because we didn't care if they were not quite as good, if a part failed and we had to wait a few days not a big deal.
Even the custom built ones, we never actually built ourselves, we just picked all the parts from an online store and ordered it assembled, usually saved us about $400 to $800 a machine compared to the production ones, but we'd never have considered using one of them for production.
Comment: I'd go with IBM (1st choice) or HP (2nd) over Dell, they're cheaper and it shows.
Answer: the issues with build v buy really come down to additional features and case size.
If you want a 1U server, buy it - really, putting the bits in is not worth the trouble, especially as you can't use standard PC bits, they all have to be low-profile, and then you also have to get the airflow right.
2U its a little better, you have more options but its still probably worthwhile to buy.
4U, you could build one - its basically just a PC case on its side. However, we now get into the additional features part: if you build a PC in a rack case, fine, but every 4U server I've worked with (eg Proliants and the like) come with hot-swap capabilities, pull-out drives, even the bits that aren't hot-swap are very easily accessed (and all screwless, you can replace a network card in seconds). In addition, there will be diagnostic indicators all flashing at you when something goes wrong (eg in the network card case, you can tell what made the server fall over because a little light was on in the slot where the network card sat).
Then there's also features like lights-out management, and diagnostic software (but you can get this yourself with an appropriate card, and stick Linux on for better diagnostics than many manufacturer's software)
Issues: with a bought server you're going to have to use the manufacturer for parts, you can't just buy any old network card to replace the old one - not if you want the same features. This means you either have a spare lying around, or support from the manufacturer - some do 4 hour onsite replacement - or you have the time to spend fiddling with the diagnostic utilities to prove to the support guys that the part really has failed (that can be a real nuisance - first thing they ask you to do is run the diagnostics, and they can take hours, only to get a replacement HDD that you know is bust because it rattles when you shake it). If you've built your own, you can buy a spare and slot it in without fuss. This is most relevant if you buy a 2nd hand server - in which case, my advice is to buy an extra one to be used for spares.
Answer: Go with a dell server. I have many years of experience with this!! Custom built servers never cut it.
Comment: +1  Much beeter choice to go brand name
Comment: While there are many reasons to use a solid brand name server the comment `Custom built servers never cut it` is massively overstating the case. Maybe you should have had someone else custom build your servers.
Answer: Even if the spec is similar, the parts quality will typically not be. Likewise, support. If you don't need reliability and support (including a repair contract) then build it yourself, by all means.
Answer: Is the local development server critical for you or just a play thing?
Business you can't afford downtime, cause it costs money. Dell provide service and matched hardware. You could build your server cheaper, but do all the parts work well together?
I would go Dell if price wasnt an issue with a hardware support. It definietly pays when your drives fail and you can turn around in a day.
It doesn't pay when your supplier of your home built server who especially ordered you that snappy scsi drive, which just now failed has to order you another. May take a few days or weeks..!! Meantime you just past your deadline on that project...whoops
Answer: If you got the budget for the Warranty/Service contract with quick part replacement, AND you don't have the on site staff that knows how to troubleshoot hardware, then go Dell.
For my stuff, the fact that I can get parts I need for a white box server within an hour at the local computer shop, is a large benefit.
Answer: I own an IT Company. I have built Servers in the past for customers. I can always beat Dells prices by quite a bit I might ad.
Remember 2 things though
1) When you build it, you warranty it, can you touch Dell's warranty without marking the Server and or Parts up in case you need to buy a spare part down the road and stay under Dells prices
2) Will that part be available when you need it 2 years from now. If you buy a dell, they make sure they have the part, in a warehouse somewhere. Can you guarantee that for 1, 2, 3 years down the road?
After building Servers for clients for a couple of years, I realized quickly that I could not compete with Dells prices, including Warranty and Service Contract.
Truthfully there is no question considering the facts. ;)
Answer: If you know what you are doing, build.
I have built many servers, both critical and not, and they always perform (per dollar) much better and more reliably than commercial (Dell, HP, etc) servers. They also are always well fitted to the task. And, they fail less often. Some "appliances" may be better purchased, like SANs, and even then, only in high-throughput scenarios. There are lots of very nice Linux-based DIY SANs out there.
Here's some thoughts:

OEMs, like Dell, are optimizing their supply + support costs. If a second-tier part costs them less to buy and support (meaning it may cost the OEM less but carries higher support costs, yet the total is less) than a top-tier part, that's what they may go with. That means you are the one left holding the server that pops while the technician comes in with the board to replace. (If you ever seen a field tech come in and say "Yeah, those things fail all the time", you'll know what I am talking about.)
If your server is critical, a four-hour-onsite warranty is absolutely not what you need. What you need is a cold spare, a hot spare, a cluster or other availability solution where downtime won't be felt by the end-users. Again, this will be cheaper if you build yourself.
A warranty is really a useless thing. You could use some of that money to buy enough spare parts to keep in inventory to cover hardware failures, and swap out the part yourself much faster than waiting for the phone drone to go through the spiral-bound book to diagnose the issue you already figured out, and then eventually send a field tech to replace it.
Also, if the hardware failure is intermittent, you are just as likely to have to fight the OEM for a long time to get a part replacement. With your own DIY server, you make the call.
The idea that OEMs keep all parts for umpteen years is an exaggeration. Usually, if you get mainstream parts, those (or like replacements) are also available on the market. Let's face it, most OEMs nowadays are JIT assemblers. If an OEM can source the part, or a similar part, likely so can you. Too add, many times when an old serv fails, you sometimes take the opportunity to move up.
Yeah, sometimes, some parts don't work well together. But, who deploys a critical server, OEM or not, without testing? If you stick with name brands, you will very rarely have issues. I've built hundreds of servers and have had few incompatibilities between parts.
You can't go from never having put parts together to building a critical, high-availability cluster without a) starting small and b) doing your research. The poeple who try "build", have a bad experience and then go to "buy" are usually people who went about it the wrong way.
One last thing: Either go with a branded OEM (like HP, IBM, etc) or build yourself. It's the middle ground that can be problematic. Fly-by-night assemblers with poor support abound.

I favor build.
Comment: To add, see Facebook's Open Compute project. They built their data center with "vanity-free", meaning non-branded, servers.


Is it necessary to add vegetables to slow cooker pot roast?

Question: I like slow cooked roast for tenderness but hate the taste of 'pot roast' vegetables. Is it really necessary to cook vegetables with the roast?
Comment: This is an interesting question. As a UK native living in the US. I always wondered this, I never even thought of the idea of roasting the meat and vegetables together as generally one wants the meat tender and vegetables crisp. Admittedly I have not used a slow cooker, placing everything in the oven in baking dishes. So this question is very interesting to me +1
Answer: If you're doing it as a pot roast, the vegetables are mainly there to deliver flavour (unlike in a stew, where they're a major part of the dish). Sometimes they're eaten, sometimes discarded (which seems like a waste to me, so I would choose vegetables I'd want to eat).  They take on a role similar to stock or a flavoured rub or oil when roasting in an oven.
On that basis you can generally omit them. One thing to watch out for though is that the vegetables can be used to lift the joint slightly, so it doesn't sit in a pool of fat*. In that case you'd want to replace them with something inert, or use them but discard them.
If you then find that the meat wants a little more flavour, you can always add a little of a suitable herb and some black peppercorns, to be discarded.

* Note that the term "pot roast" is used broadly, sometimes adding significant amounts of liquid  and sometimes not. I don't know if this is a regional thing, but my answer is meant to cover both types.
Comment: It's also easier to throw everything in one pot and not have to cook separately.  If you don't know the exact meal time, a slow cooker can be automatically keeping the whole meal immediately ready for serving when the unknown time arrives.
Comment: @Keeta - That's precisely how I use the slow cooker. Meat and veg get chopped up into the same bowl, then just left to cook all day. On your return you've got a one-pot meal with meat and two veg
Comment: @Richard After second thought, I decided it may be better to make it an answer all to itself, rather than updating this one.
Answer: To directly answer the question as it was originally posted before edit:

why does everyone add vegetables to slow cooker pot roast

To save time
Longer answer:
It is not necessary to cook vegetables with meat.  The vegetables do add flavor (a flavor that many appreciate) but I don't feel that is the main reason.  Many people add potatoes to the pot roast while cooking.  Potatoes don't add any appreciable flavor.
Instead, in my case I do it to save time.  Sometimes I will plan my lunch meal on Sunday and put things in the pot to cook.  By having everything in one pot, when I return from morning services I can just pull everything out of the pot and put it on the table.  Based on services ending just about noon and the travel time home, this removes the need to wait for food to be prepared for the lunch meal.
It's also easier to do.  Rather than put only the meat in one pot and then create more dirty dishes by cooking the vegetables separately, I can put them all in one pot and make cleanup time shorter.
So, to answer what I feel is a very opinion based question, "to save time".
Comment: You also don't have to add everything at the same time -- unless you're in the 'set it up and leave it alone 'til it's done' situation (eg, while at work), you can add some of the vegetables after the meat has cooked for a few hours so they still retain their texture.  (frozen peas to just warm through add good texture)
Answer: I agree about those vegetables. Julia Child has a great solution.
Julia Child only adds aromatics like onions, celery, carrots, and garlic (never potatoes) to a roast in her pot roast recipes (see Beef Burgundy). Once the roast is finished, she strains-out the aromatics because they are now tasteless and mushy. She then sautes pearl onions in butter and adds those in just before serving. If you want potatoes, she cooks those separately or adds them to the pot roast about 2 hours before the roast is finished cooking.
Google "julia child beef bourguignon" for her recipe and many variations. BTW, her recipe is delicious, it is worth any extra time or expense to try at least once in your life.
Answer: Sometimes it's to lift the meat up so it's not cooking directly on the bottom of the pot. In any case, if you wrap your meat in parchment paper you can roast it in the oven and get pretty much the same effect as a pot roast.
You still need to add aromatics like onions to your roast if it's in the recipe though. That's a flavoring issue.
Comment: Agreed on the lifting it -- I always either place a couple of carrots (not necessarily chopped ... just two peeled carrots) or some onion across the bottom so the meat is surrounded by liquid.  I think it's more important for ones with metal vessels (multicookers, slow cooking in a rice cooker, etc.)


Where can I get the Beijing ring road traffic data (speed of vehicles)?

Question: In some papers, it is used the Beijing ring road traffic data.
I searched the internet, But I didn't find the dataset.

For example, in a paper, it is noted:

"Beijing is the capital of China and one of the largest cities in the world. At present, Beijing is encircled by four two-way ring roads, that is, the second to fifth ring roads, and has about 10,000 taxis to serve its population of more than 21 million. These taxis are equipped with GPS devices that upload data approximately every minute. The uploaded data contain information, including car positions, recording time, moving directions, vehicle travel speeds, etc. The data were collected from 1 May 2015 to 6 June 2015 (37 days)."

How can I get them? Is there a site to download it?
Or do you know a good data set instead of it that contains the speed of vehicles?
Thank you
Answer: A week's worth of GPS traces from 10,357 taxis in Beijing: http://research.microsoft.com/apps/pubs/?id=152883


Can a society with no electricity develop modern weaponry?

Question: If electricity didn't exist or behaved differently, would a society be able to develop and reliably produce modern-level guns and other weaponry, like assault rifles and missiles?
Comment: This entirely depends on exactly what you mean by "modern-level gun" and "missiles". Too broad.
Comment: Can you clarify what you mean by 'modern'? Contextually what you're asking could mean either present day or the late modern period (19th century through to the end of WW2)
Answer: This entirely depends on exactly what you mean by "modern-level gun" and "missiles".
Weapons are very simple. Stamp out some metal parts and mix explosives and that is it. The "assault rifle" is a century old - the basic principles are even older and all that is updated is some better metallurgy and more precise machining (which just takes technological refinement).
Missiles are a tricky question because it depends on exactly what you mean by "missile". Rockets predate electricity by hundreds of years, but it would be absurd to expect modern guidance systems (by definition electronics are not possible without electricity). You would need a much clearer indication of what you intend here - a metal tube self-propelled by combustion is easy as soon as you have enough basic chemistry to make an explosive, while infrared or radar guidance packages are clearly not within scope.
As a general rule, think about the weapons used in the first world war - you generally don't need electricity for what was seen on the battlefield (obviously telephones are out but I'm referring to what the typical soldier actually carried), and any advances beyond that which are just refinements on the same theme are likely possible as well.
Comment: Guided missiles would still be possible without electricity. https://en.wikipedia.org/wiki/Project_Pigeon Now while this did use cameras and screens to help the pigeon pilot the missile, it is conceivable that an animal could be trained to fly it without electrical aid.
Answer: Yes to the guns,  even an "assault rifle" is just a machine gun.  This is an exercise in metallurgy, engineering, and chemistry.
Missiles are much more complex, and imply some sort of guidance system.  Germany developed the V-1 flying bomb, using a pressurized-air powered gyroscope based auto-pilot.  A "vane anemometer" was used as a odometer, to determine when the bomb had traveled far enough, and should descend.
Approximately 10,000 of these bombs were fired at England, and about 2400 reached London.  So they do have some level of operational effectiveness.
Comment: This of course limits the "missile" to attacking ground or fixed targets,  Hitting moving targets, "air-to-air" missiles would require some sort of detection and tracking for which a non-electric alternative is unlikely
Answer: I'm going to focus just on firearms here as that's what I know.
Let's look at production, as it's kind of pointless to design something if you can't produce it.
What goes into making a 'modern' firearm?
Metal is mandatory if you want it to be portable (as Mythbusters has proven, you can make a cannon out of a big log, but that's way too big to carry).  For most mass produced guns, steel (often 4140, but occasionally other alloys) is the material of choice, because it's the cheapest option that meets all the requirements.  Steel has, of course, been produced by humans to varying degrees of quality for millenia, so that's one thing off the list.
Most modern firearms also have the metal components specially treated to resist corrosion and abrasion.  Standard methods include, in relative order of development:

Bluing: Is a simple passivation process that produces a layer of magnetite (black iron oxide, or iron (IV) oxide) on the outside of the metal part.  There are four main methods that have historically been used, but none of them 'require' electricity.  The methods that involve heat (also the most efficient ones) can be made more efficient through use of electricity (electric heating is generally more efficient than using something like a forge), but still don't require it.
Parkerizing: Also known as phosphating, is similar in many respects to bluing, but produces a layer of iron (III) phosphate.  In most cases, it produces a distinctive satin black finish (if you've ever seen a Glock in real life, you would recognize the finish).  Functionally, parkerizing is low temperature heat catalyzed chemical process, and thus also does not need electricity except for improving efficiency.
Ferritic Nitrocarburizing: Also known by the trade names Tennifer and Melonite, this is much more complicated, and can't be simply explained by the production of a single compound.  Unlike the other methods, which are simple enough to not require particularly precise control, ferritic nitrocarburizing is an extremely sensitive process.  While the salt-bath method is technically possible without electricity, it requires sufficiently precise control that it pretty much needs electricity for any kind of reasonable results.  The other methods all require electricity.

So, the protection of the metal parts would have to resort to (less reliable and less effective) older techniques.
Plastic is also very common in many modern firearms.  Usually, it's used in the grip, the trigger, and other components that don't typically take heavy shock loading when the weapon is fired.  With almost no exceptions though, pretty much anything you can find on a modern gun made of plastic could be made of either metal or wood, so this isn't an issue.
The next potential issue is tolerances.  Manufacturing of a firearm needs to be reasonably precise for a couple of reasons:

Accuracy.  Lack of precision results in weapons that aren't consistently accurate (that is, the variance between individual weapons will be larger).
Repairability.  You have to have rather precise (but loose) tolerances if you want to have drop-in replacement parts like most modern designs (eg, AR-15's or Glock's) do.
Safety and reliability.  If things are off by too much in certain places (for example, the chamber dimensions), you end up with a gun that isn't reliable or safe.

Effectively, you need modern tooling to make modern guns.  This is technically possible without electricity, it's just insanely difficult, and you won't' be able to get quite as precise as modern tooling is (modern stuff isn't limited by the precision of human visual acuity or fine motor skills, without electricity you do have those limitations).
So, given this, so far it's possible, but probably not for mass production.
Next you have to consider ammunition.  The same requirements for precision tooling apply here, except it will mostly just hurt your accuracy.  As far as materials, standard ammunition consists of:

Lead:  Easily doable, people have been casting lead since before they could produce steel.
Brass (for the casing): Also pretty easy, though the exact alloy is important.  For quality on par with modern designs, you need at least WWI era metallurgy skills, possibly WWII.  Alternatively, steel works, though it's potentially bad for the chamber if it's too hard, and aluminum works too (but that functionally needs electricity to get any realistic quantity).

And optionally:

Copper: Used as a casing for the bullet.  Not mandatory, but makes it easier to clean firearms after use and reduces metal buildup in the barrel.  This is just as trivial as the lead.
Some particularly hard metal: Armor piercing rounds have a special hard metal core.  Traditionally it's steel, but it could also be almost any other hard metal.  Modern rounds may use titanium, tungsten, or even depleted uranium.
Wax: Used in some ammunition to waterproof it.

So, basic materials are covered.  That leaves the propellant, and the primer.
The primer is easy.  Pretty much any metal styphnate or fulminate will work, as will any other shock-sensitive compound, so you have lots of options, most of which were readily available long before electricity.
The propellant is a bit trickier.  Modern guns use smokeless powder, not classical black powder.  Smokeless powder is complicated.  For those who care about the specifics, the Wikipedia page is a good starting point.  In short though, it's possible to make smokeless powder without electricity, but it's expensive.
Overall, is it possible to produce a 'modern' firearm without electricity?
I'd say it's a definite yes, but it's not going to be cheap.  Looking to some modern designs that use no plastic, you're looking at about 800-1000 USD for a decent semi-automatic handgun with electricity involved.  I'd expect that to almost double without electricity, and for the ammo costs to almost triple.
OK, what about more specific stuff like assault rifles?
I'm going to assume you mean a military assault rifle, not the politicized name for a 'scary weapon'.
An assault rifle is typically defined as a long gun (a firearm designed to be fired from the shoulder, thus having a stock) with selective fire capabilities (it can be switched between semi-automatic and full automatic, and usually also offers a burst-fire mode too in modern designs) chambered in an 'intermediate' cartridge (something less powerful than classical battle rifles like the FAL or M14, but more powerful than handgun cartridges).
The first 'modern' assault rifle was the StG 44, a firearm developed by Nazi Germany in 1942.  If you want to go even further back, the 'first' fully automatic rifle was the Italian Cei-Rigotti, developed in 1900, and the first 'practical' fully automatic rifle was the American M1918, developed in 1917 just before America's entry into WWI.  All three of these were originally produced without requiring any electricity.
So, yes, most modern assault rifle designs are possible without electricity (though do note that some designs that never really caught on, such as the H&K G11 would not be possible without electricity).
OK, what about all the stuff other people are saying you need electricity for?
In most cases, a lot of the things other people are claiming electricity is needed for for the production of firearms just need it for efficiency.  Namely:

Computer Aided Design:  Nope, not mandatory for making a modern firearm.  Drafting is a lost art at this point in our world, but there's no reason it would not be on par in terms of precision with what we can do with CAD software today in a world without electricity.  Realistically, some designs would be out without CAD software, but most of them either never caught on in our world, or require electricity anyway (see for example the aforementioned H&K G11).
Electric machining tools: Also not needed.  Many such tools either exist in some steam powered form (for example, power hammers or lathes), or were developed when electricity was already ubiquitous but don't necessarily need it (for example, drill presses or milling machines).  In reality, there's essentially nothing except the barrel and chamber on a modern firearm that you can't just cast instead of machining it, and the barrel can easily be machined by hand (in fact, people still do this in some cases, though it takes significant skill to get the rifling right).
Alloying: OK, this one is actually possibly a problem if you want really 'modern' stuff like vanadium or titanium alloys, but such alloys are unnecessary for modern style firearms.  As I mentioned above, most stuff is some form of tool-steel, usually 4140 or 4150.  4140 is a mid-carbon steel (that part is easy) with traces of chromium, molybdenum, manganese, and silicon, and low sulphur and phosphorus content.  Overall, that is actually doable with old 1800's technology, with the appropriate knowledge, though the level of precision for the trace components would be well outside SAE tolerances.  It doesn't need to be 4140 though.  Any alloy with reasonably similar mechanical properties will work, provided it can be case hardened, which encompasses a wide selection of alloys.

So, yeah, overall, still very doable, just not at levels of production we have, and things would likely be significantly more expensive.
Answer: Without electricity you will need to redesign the history of Humankind. For example: Tomas Alva Edison invented the light bulb in 1879, and in 1800 Volta invented the battery. You need to redesign the civilization without electric light and without batteries, so there will never be electric motors, or telephone, or any radio communication. Perhaps the most widespread machines on industry could be steam powered, however, I don't see why weaponry could not evolve. Even rockets or assault rifles, just with a slower development. We humans are very creative with our inventions. Actually we could built clocks before the electricity (see this Forbes article: https://www.forbes.com/2008/02/28/oldest-work-clock-oped-time08-cx_po_0229salisbury.html#49f13c0531f3
But definitely, everything will be sloooooower, starting with the communications: email, whatsapp or the internet will never exist. And with that, all the society evolution.
Comment: You only have to redesign the history of humankind up to the 19th Century. It's arguable whether what's happened since then is "society evolution", but that's beyond the scope of this comment
Comment: It isn't just obvious things that directly use electricity but materials (such as aluminum) and tools (such as electric arc furnaces) that require electricity. So yeah, there are a lot of side effects to not having a fundamental technology like electricity.
Answer: While large-scale manufacturing and precision manufacturing is going to be hard, any chemical based weapon wouldnt be impossible. R&D behind it would take much much longer though.
Answer: I'll disagree (in principle) with some of the other answers, at least as far as missiles go. (Again) in principle, some forms of guided missiles might be possible.
There exists a form of logic called fluidics which uses the flow of a liquid or gas to perform logic operations. This could be used to control a missile in flight, since it would use the same high pressure fluid both to perform computations and to drive the output actuators.
The concept was being developed at GE in the mid-60s as means of making smart artillery shells, since the final processor would be extremely robust, although some problems were experienced with Coriolus forces affecting the fluid flow. The whole idea was eclipsed by the explosive growth in electronics capabilities, but seems to be poised to make a modest comeback in some applications.
However, this only applies to missiles which are self-contained in flight. Anything needing either a command link or radar is going to be out of luck. Although even then, people's ingenuity makes absolute pronouncements hazardous. Homing doesn't actually require radar, necessarily: there was a peculiar military program which attempted to use pigeons as suicide pilots.
Answer: Missiles use electricity, They carry a high voltage battery to power the onboard gyrostabilizers and whatnot. Rockets however, can be ignited via a percussion cap and since theyre unguided they need not have any onboard power supplies.
Assault rifles dont require electricity to be made, even a lot of modern ones could be produced in an old fashioned steam engine driven machine shop. They will just take about twice as long and cost twice as much to make.
Comment: As the V-1 (and for that matter, the older Congreve rocket) demonstrate, a missile doesn't *need* electricity.  It just makes the guidance system smaller and more accurate.
Comment: The V1 was indeed very crudely guided, but in fact did have electrically powered mechanisms on board to do so. You are correct that rockets do not require electricity, but any guided missile will. remeber, the rule of thumb to tell the difference between a missle or a rocket can be summed up thus "Missiles are smart, Rockets are dumb." If its guided its a missile, if its not its a rocket.
Comment: @TCAT117 A missile guided by a compass and a clock (which is enough to have it crudely find a target) fits, IMHO your definition of a *guided missile* - There's absolutely no reason you couldn't build such a guidance system without electricity.
Answer: There are actually not many electrical components you couldn't replicate using pneumatics or hydraulics. The only area that might lead to problems is the capability of electricity to produce, emit and register light, heat or other radiation or trigger chemical processes. You could even build a pneumatic or steam-operated computer, accepting that it is going to be much bigger, clunkier and much more susceptible to mechanical wear than its electrical counterpart.
Some technologies like electrolytes that are typically used for metal plating or the production of metal alloys can be replaced with pure chemical processes, albeit at the cost of effort and time (you can produce Aluminium, for example, without electricity - See potassium reduction).
Weaponry as such is pretty simple technology - an assault rifle does not need electricity, neither for operation nor manufacturing. Even guidance systems for missiles and rockets can be purely mechanical (even based on clockwork technology, maybe, early torpedo guidance systems were purely mechanical), but might have some limitations on sensor technology and no obvious method for a remote control without a radio technology.
You would face a lot of problems that you wouldn't face when having electricity available, but keep in mind people normally don't miss things they don't even know they exist...
The fact that human technology was never forced to invent solutions that work around missing knowledge about electricity (which I, BTW find highly improbable, but that was not your question), doesn't mean that such workarounds do not exist.
Answer: I think it all depends on your scenario. Electricity exists by itself and it's known since 2700BC and Thales made experiments with electricity as early as 600BC, so in any case what you mean is "ways to create or manage electricity as we know it". Or do you mean ELECTRONICS?
Think about this: electronics needed for missile control require electric circuits. Do you know which other thing require (and has) electric circuits and impulses? Neural Networks. So, as you can see, you don't need a society to understand or manage electricity, it just exists inside us, even in animals. However, you need an understanding of the methods to manage it to create electronic components!
You also mention it could behave differently, so that allows us to think in scenarios where a society developed other methods. Maybe gravity, magnetism, fluidics, quantum chemistry, quantum computing, electrochemistry, advanced thermal energy, some kind of unknown physics, even magic, why not? Electronic devices use energy, so as long as you can create energy from some source, you'll be able to develop devices. It doesn't matter what kind of energy you create as long as it is enough to make the device work
Furthermore, I see fluidics was already mentioned, so here you have some uses of fluidics:

Fluidic injection is being researched for use in aircraft to control
  direction, in two ways: circulation control and thrust vectoring. In
  both, larger more complex mechanical parts are replaced by fluidic
  systems, in which larger forces in fluids are diverted by smaller jets
  or flows of fluid intermittently, to change the direction of vehicles.
  In circulation control, near the trailing edges of wings, aircraft
  flight control systems such as ailerons, elevators, elevons, flaps and
  flaperons are replaced by slots which emit fluid flows. In
  thrust vectoring, in jet engine nozzles, swiveling parts are replaced
  by slots which inject fluid flows into jets. Such systems divert
  thrust via fluid effects. Tests show that air forced into a jet engine
  exhaust stream can deflect thrust up to 15 degrees.[citation needed]
  In such uses, fluidics is desirable for lower: mass, cost (up to 50%
  less), drag (up to 15% less during use), inertia (for faster, stronger
  control response), complexity (mechanically simpler, fewer or no
  moving parts or surfaces, less maintenance), and radar cross section
  for stealth.
  This will likely be used in many unmanned aerial vehicles (UAVs), 6th generation fighter aircraft, and ships.

In short: quite honestly, your question has to be rephrased and you'll need to provide details about what level of scientific development did your society achieve. But as far as it goes, the answer is: YES, it's possible
Answer: You could try to train animals to steer missiles.  Or as in an Issac Asimov story you could use humans.  Then again you could use use sex starved males... :-{
Comment: Welcome to worldbuilding.SE.  When you have a moment, please take our [tour] and visit our [help] to learn more about us.  This doesn't actually answer the OP's question (can a society with no electricity develop modern weaponry?).


Do we really have a free will?

Question: If I have a free will, but I don't get to choose to not have a free will, I just have to accept what is forced on me, doesn't that imply that I don't have a free will?
Comment: Free will in an interesting sense is not about being able to do whatever you want. I was born, I have to accept that - that's a terrible argument against free will, though. Interesting free will amounts to our metaphysical power within the causal process, vaguely speaking.

In any case, your question is extremely broad. I don't think we can tell you whether or not we have free will because there's still plenty of disagreement about it. Have you taken a look at [other questions about free will](http://philosophy.stackexchange.com/questions/tagged/free-will?sort=votes&pageSize=50)?
Comment: I do. Do you? Perhaps it's a personal choice.
Comment: Oh, then you say a more interesting definition of free will, is not to be able to choose whatever you want to choose. But to be able to choose from what is that is on the table?
Comment: Possible duplicate of [How can someone have a free will if they act based on their experiences?](http://philosophy.stackexchange.com/questions/5828/how-can-someone-have-a-free-will-if-they-act-based-on-their-experiences)
Answer: Free will is not the ability to do whatever you want if was like this then try to fly!
Free will is the ability to make a choice. If this wasn't true we are in trouble:

Man has free-will: otherwise counsels, exhortations, commands,
prohibitions, rewards, and punishments would be in vain.
man acts from judgment, because by his apprehensive power he judges
that something should be avoided or sought

This is just an extract but have a read on I answer that of: Summa Theologiae, Question 83, Thomas Aquinas
He continues:

Now particular operations are contingent, and therefore in such
matters the judgment of reason may follow opposite courses, and is not
determinate to one. And forasmuch as man is rational is it necessary
that man have a free-will.

click on the link to examine in depth the question on the relative oppositions.
Answer: As another answer correctly pointed out, the mere fact that we don't have the option to do something does not exclude the possibility that we have free will. It is not a necessary condition of free will that we can choose to do all possible actions, but it is a sufficient condition that our set of possible choices is greater than one.
As to the more general question, there are three schools of thought regarding free will. Those are libertarian free will, compabitilism, and hard determinism. ("Libertarian", in this context, is not to be confused with "libertarianism" as a political philosophy.) Here is a brief description of the three positions, and then I will elaborate on them further.
Libertarian Free Will: We are free to make choices, and are not bound by the deterministic laws of of the universe. That is, for any decision we make, we "could have done otherwise".
Hard Determinism: We are not free to make choices. The universe is bound to deterministic laws, and that includes everything that composes what we are. We "could not have done otherwise".
Compatibilism: We are free to make choices, but we "couldn't have done otherwise". We are bound to the deterministic laws of the universe, but this is not incompatible with free will.
The thing that differentiates libertarians from the other two categories is the notion that you "could have done otherwise". That is, if we reversed the universe back to the identical state that it was in before you made a certain decision, could you then do otherwise? Or would you be doomed to making the exact same decision again? Libertarians believe you could have done otherwise. Hard determinists and compatibilists do not. The latter two accept that the universe is bound to deterministic laws, including everything that composes you. Libertarians reject this claim. According to the libertarian, there must be some sort of agent that has a fundamentally unpredictable means of making a choice, and that choice directs the actions that you take. According to the libertarian, if everything you examine about the human body and brain appears to obey deterministic rules, this would imply that there is something as of yet undiscovered about the human that dictates decisions. A famous libertarian philosopher who voices his justifications for such claims is E. J. Lowe.
The thing that separates compatibilists from hard determinists is a lot more subtle and hard to pin down. To most people, the notion that you "couldn't have done otherwise", and yet still have free will (as compatibilists believe) is unintuitive. But as a compatibilist would argue, free will does not necessitate the option to have done otherwise. After all, it is still true that we have a mind, and that that mind deliberates, and that many (not all) of our actions are a result of said deliberation. Had the result of the deliberation been different, we would have performed different actions. This observation does not require that the result of the deliberation could have been different. This is not an obvious concept to grasp, and there are plenty of arguments to help absorb the claim of compatibilism, with examples such as Daniel Dennett and Sean Carroll who have famous thoughts regarding this subject. In fact, compatibilism is the position most commonly held among academic philosophers.
Hard determinists, like compatibilists, believe that we are subject to the deterministic laws of the universe, and thus our actions are fundamentally predictable (even though they're not practically predictable). However, unlike compatibilists, they reject the notion that determinism is compatible with free will. To the hard determinist, there is no notion of free will that can sensibly describe a world in which you could not have done otherwise. In many cases, they consider the compatibilist free will notion to be one of an equivocation of the term.
Answer: In India there is no direct philosophy of free will but the notion of Karma which essentially means you reap what you sow or you eventually have to face result of your karma. Hence, you are free will to decide what action would you take but your thought of any action is predefined by the fact of Karma that is every wrong or unethical action is also going to result the same. The concept of karma why, how and what and the motivation behind is discussed in Bhagvad Geeta
Comment: This explains why, in karma yoga societies, sufferers of misfortune have to deal with stigma because karma says the suffers' ordeal is punishment for the sufferers' own sins in earlier lives. As a matter of fact, this is exactly the sort of thing the oppressors say to the downtrodden where karma yoga philosophy is endemic: those who profited from inflicting cruelty upon others have none but their own good virtue to thank for; those who suffered abuse have to live with black curses on their backs for misdeeds they have no memory of but have to inferred from their sufferings in this life.
Answer: As we live in a generally deterministic universe, with the only unpredictable parts of nature being those that we're unable to fully comprehend, it seems reasonable that free will is an illusion, and that our choices are all predetermined.
I look at it this way: As humans, we will always pick what we believe to be the best option, considering all the options available for us to consider. How we interrogate these options differs on an individual basis, due to hormones and other chemicals, as well as slightly differing levels of information, but in the moment we are picking the choice which we believe is most likely to get us what we desire. This means it's predetermined, as if you gave the same person, in the same mental state, the same options, they would always make the same choice (this is debatable, however I've seen no compelling evidence to show otherwise).
This seems reasonable to me, and means that free will is really just an extension of our desires. So, do we have free desires, or are they predetermined too? For this we need to think about where our desires come from. Lets take a basic example: Hunger. Hunger is a predetermined desire, it will always occur, due to the body's natural decay. Does that mean deciding to eat is also predetermined? I would argue that it is, and that the illusion of free will exists in the choice of 'what' to eat - however, if we only had one option, we would always take it, so I believe the choice of foods is just a more nuanced predetermination, which I'll elaborate on now.
I believe more nuanced desires are also predetermined, but due to the complexity of our senses and the scope of our lives, they appear to be random on an individual basis. I think we can all agree that a bacteria doesn't have free will, nor does a Jellyfish which doesn't even have a brain. A plant, too, doesn't have free will, all 3 are automatically responding to stimuli from their environment. Extrapolate from this, a bit, and you start to wonder: does a fly have free will? What about a bird? These creatures are all generally agreed to be operating on instinct, which would imply no free will, however once you reach cats and dogs, they start to seem more than the sum of their parts. Does a cat have free will, because it decides when to come and go? Or is it again, just operating on instinct? I'd argue the latter, based on the trend, and a lack of dogs committing suicide or acting irrationally - there's always an explanation for why a crazy dog is crazy.
So, what makes us more advanced than a dog? Sentience, perhaps? This is where it gets tricky, we consider ourselves more advanced than a dog, based on a measure that we created, and a concept of thought mechanisms that we invented ourselves. However, if every aspect of nature, up to humanity, is devoid of free will, the more rational possibility is that humanity invented the concept out of self-centeredness (the core of the evolutionary process).
Here's a hypothetical (or not) situation: When free will first arose, what drove the thoughts of the individual who was first blessed with it? Did they make their choices based on what was most likely to benefit them, and further their objectives? Did they still eat when hungry? Drink when thirsty? Sleep when tired? I believe that free will is simply an illusion, created when the complexity of predetermined thoughts became too much. An automated system, analysing itself, and deciding it is not automated, seems much more reasonable to me than a spontaneous, exclusive power of the human mind, allowing it to ignore the deterministic nature of the universe, despite the fact that galaxies, stars, planets, elements, atoms, cells, bacterium, plants, and animals all appear to follow these laws.
Quantum mechanics is the one possible out - if the quantum level is not deterministic, there's no reason for the brain to be, but I would put forth two points:
1) we have no guarantee quantum mechanics are not deterministic, we may not be advanced enough to understand their determinism
2) if quantum mechanics are truly random, all that gives us is the potential for random chance affecting our decisions - not exactly free will, any more than a computer program has free will if it is randomly generating lines of text.
I'll leave it at that, this was kind of a brain dump so any clarifications I can give, I will, if a comment is left.
Comment: If someone would like to tell me which parts of this they have issue with, and believe require additional references, I will be happy to add them, however from my perspective it is fairly self explanatory, so vaguely requesting references is pointless.
Comment: I wouldn't consider QM an out at all. Even if there is an inherent randomness to it, that randomness has a given probability distribution which can be calculated entirely from the physics of the system. There is no notion of "agency" in that distribution. Also, the fact that there is a distribution doesn't imply that "one of these things will happen". Indeed, all outcomes are happening at once, and we can calculate exactly how the superposition of those states evolve in time. In that sense, QM is still deterministic and we *are* advanced enough to understand that level of determinism.
Comment: Thanks for the comment, I'm inclined to agree with what you're saying

Do we really have a free will?


What is the difference between "dry rope", "non-dry rope" and "dry core rope"?

Question: I've just recently gotten into climbing and so far the ropes have always been provided for me. I'm starting to look at getting my own rope for outside climbing, and am confused by some of the terminology. Many of the ropes say "dry rope", "non-dry rope" or "dry core rope". What do these terms mean, and what are the conditions or types of climbing in which you would prefer one or the other?
Comment: The variations of dry treatments, such as "dry core", may depend on manufacturer, rather than being amenable to a strict general definition.  E.g. Beal has Dry (sheath only) and Golden Dry (sheath and core), Mammut does similar, and Edelrid has "Dry Shield" (sheath and core).  Sterling has DryCore which apparently is applied to the core of every rope, and the ones they sell as "Dry" get a second dip to treat both sheath and core.
Comment: @requiem you should post that as an answer, basically explaining that "it depends on the manufacturer". It looks like all of the answers so far have slightly disagreeing definitions, and the real answer is that you really have to look it up for each one.
Answer: There are many phrases that you will find concerning dry treatment of ropes, but they can all be easily related to your three categories:
non-dry rope
This rope has no treatment to repel water. Consequently it absorbs the most water and thus getting heavier. Wet ropes also loose some of their dynamic properties, so falls will get harder. As it is the cheapest variant, it is well suited for indoor climbing and also fine for sports climbing (you usually don't sport climb in heavy rain).
dry rope
The sheath of this ropes has a dry treatment. This means that the individual fibres are treated to repel water. As the sheath is on the outside, this is already a good protection against water. This is a goto rope for any multi-pitch, alpine climbing and mountaineering (except in case of heavy icing, see below).
dry core rope
Both the sheath and the core fibres are dry treated. This is an additional protection against water that may pass the protected sheath. As the core has much more fibres than the sheath, this is more expensive. This rope is for ice-climbing and mountaineering in mixed conditions. In those activities it is pretty much possible for a wet rope to freeze which makes it unusable, so it is vital it stays dry.
Comment: Your explanation of "dry core" is that the sheath and core are dry treated; shemseger's explanation is that only the core is treated. Could you provide some reference for this? I'm not sure which to believe; either seems plausible. Thanks.
Comment: http://www.climbing.com/gear/2012-gear-guide-how-to-buy-ropes/
Look for the "Dry treatment" section. It is also kind of common sense, that the outer layer is made waterproof first (you don't wear your rain jacket below the wool pullover). The term dry core rope is kind of misdirecting, how it is often the case with such "catchy" product specification terms.
Answer: What are the differences?
The different ropes basically differ by how they have been treated to handle water: non-dry ropes (although I've never seen that mentioned explicitly) have no special treatment at all, dry ropes have only the sheath, treated with some water-repellant, while dry core ones also have a treatment for the core. In both latter cases, the dry coating does not make the nylon fibres immune to water, it just prevents that they suck up any water immediately.
Why does it matter?
A rope can soak up a significant amount of water that will make it heavier, worsen handling and will strongly reduce the dynamic strength, i.e. the fall energy it can absorb – the figure of 70% loss in strength is around. Besides that it does also not matter much whether it is soaking wet or just damp, as stated here

Tests have shown that just being splashed under a shower had comparable strength reduction to ropes that have been fully soaked; i.e., the amount of strength and performance loss is nearly equal for both a damp rope and a fully soaked rope.

So basically everything more than some single rain drops may be too much. But it should also be mentioned that these issues are temporary, so once you have dried the rope it does behave as before. The only exception is the case, where the rope has taken a lead fall while wet, in that case it may have been significantly weakened and it is better to discharge it:

Any fall on a wet rope causes more damage, so its future performance (even when dried) is compromised. (source)

Which treatment for which use case?
Concerning the question which rope to use for which activity: I would say for 90% of climbing activities you are fine with a non-dry one and a rope with dry treatment is just wasted money since the treatment will wear down as you se the rope.
So as long as you use your rope for activities where you don't expect to have it exposed to water or snow for prolonged times, a non-dry rope is enough. This covers basically indoor and outdoor sports climbing since normally you are only performing those activities in fairly good weather and even if some unexpected rain shows up it's no problem to rappel down one pitch on a rope that got a bit wet from the rain.
For multi-pitch rock climbing, it is somewhat debatable if a dry coated one is just nice to have or essential. It depends a bit on what type of climbing you are doing there. If you are just doing some multi-pitches in a developed climbing area where you just rappel down some pitches, get to your car and rush home if it starts to rain, you won't need a dry rope. You can do all this also on a wet rope  – the handling will be a bit uncomfortable but it works. If you are climbing in some alpine area where there is a chance that you have to rappel some ten pitches in a heavy rain storm on a soaking wet rope with cold water running down the rope as the rappel device squeezes it out of the rope and the whole rappel taking twice as long since friction has increased dramatically and then you have to hike out another one or two hours while carrying your soaking wet rope that is more than twice as heavy as in dry condition, you will condemn yourself that you wanted to safe those bucks on a dry coating. If you plan to climb on the last few pitches to the top instead, you should make sure you have a rope with dry coating, since a not coated one might break under a lead fall (source).
Where a dry coating is quite inevitable is everything where it is clear a priori that the rope will come in contact with water and/or snow, i.e. ice climbing and glacier hiking.
In short
Rappelling on a wet rope is ugly but still OK – having a lead fall into a wet or even damp rope may be critical. So whenever there's a chance that you will have to climb on (and not just rappel down to go home) even after your rope has been in contact with water, you should invest the additional money for a dry coating. On the other hand, it's wasted money for the most widespread cases (sports climbing) where you just stop as it starts to rain.
Comment: This answer is offering potentially dangerous advice.  Wet ropes have significantly reduced dynamic strength, even if only splashed with water.
Comment: @ShemSeger You're basically right. Although I did never recommend to climb on on a wet rope but only mentioned rappelling I did some edit to clarify on this.
Comment: @ShemSeger correct. http://www.climbing.com/skill/wet-rope-myths-debunked/ *Rope companies do not recommend falling on a wet rope, which may have its dynamic performance reduced by up to 70 percent when wet.*. That said *Modern dry-treated ropes are a bit better, with dynamic performance reductions of about 40 percent* too
Answer: Wet ropes are heavier and provide less energy absorption, which is a big problem when you can't avoid wet conditions and you rely on your rope to protect you if you take a fall.
Dry ropes have been impregnated with a fluoropolymer-based solution to prevent them from absorbing as much water as possible. Use of dry ropes is essential for alpine and ice climbers, or for glacier travel. Non-dry ropes will suffice in other conditions, although a dry treatment is also useful for reducing wear from abrasion, and it provides better energy absorption.

There are at least two types of dry ropes, those that have a single dry coating, and those that are "double-dry", they've had both their sheath and their cores coated. Single dry ropes typically are only coated on their sheaths, but the coating tends to wear off quickly, so some manufacturers use a single treatment that penetrates to the core.
The terminology doesn't seem to be consistent, some distributers only categorize ropes as dry or non-dry, others add categories like dry-core, dry core and sheath, or 2X dry. But the difference is just how many times the ropes are treated to be water resistant.

1 MEC - Choosing a climbing rope
Image: Outdoor Blueprint - Climbing Ropes
Comment: @nhinkle - The difference appears to depend on the manufacturer.
Comment: Your explanation of "dry core" is that only the core is dry treated; imsodin's explanation is that the sheath and core are treated. Could you provide some reference for this? I'm not sure which to believe; either seems plausible. Thanks.
Answer: Dry ropes and dry-core ropes are very similar. Basically they've been treated so they repel water (this does not make them water proof). "Wet" ropes have no treatment.

What do these terms mean, and what are the conditions or types of
  climbing in which you would prefer one or the other?

If you live anywhere where it rains and you plan to climb (dynamic loads), I'd get a dry treated rope. Almost everyone in the UK has some kind of dry treatment on their ropes.

So why do this?
Basically a wet rope is not a good thing.

Rope companies do not recommend falling on a wet rope, which may have
  its dynamic performance reduced by up to 70 percent when wet. Modern
  dry-treated ropes are a bit better, with dynamic performance
  reductions of about 40 percent, depending on the type of dry
  treatment. Any fall on a wet rope causes more damage, so its future
  performance (even when dried) is compromised.

So how wet does a rope need to get for it to be compromised?

Tests have shown that just being splashed under a shower had
  comparable strength reduction to ropes that have been fully soaked;
  i.e., the amount of strength and performance loss is nearly equal for
  both a damp rope and a fully soaked rope.

This is where the dry treatment comes in. A dry treated rope will likely pass the "shower test" where a wet rope won't. But if you submerge either into a pool for any length of time and they could be compromised.
So I shouldn't get my rope wet?
No, getting it wet isn't a problem, getting it wet and then using it is. It's actually a very good idea to clean your rope once in a while.

If there were any long-term effects of soaking and then drying ropes,
  rope manufacturers would not recommend washing them at all. However,
  the rope has more elongation and is more susceptible to abrasion when
  wet, which can reduce the cord's ability to absorb future impacts

Sources
TL;DR
The important thing to remember is that a wet rope is a compromised rope. Dry treatment will help prevent it getting wet but if it get's wet, don't use it for anything dynamic.
If you do fall on a wet rope then you should retire it immediately:

So if it underwent any major trauma when wet, retiring the rope is
  recommended.

A note on gorge scrambling.
It's common (in the UK at least) to use ropes while gorge scrambling. This is fine as they are typically confidence ropes and will be under static load (as opposed to dynamic falling loads)
Answer: This question has already been well answered but I would like to address a slightly different point:
What's the problem with a wet rope anyway?
Dynamic climbing ropes are, to the best of my knowledge, universally manufactured from Nylon 6 or Nylon 6,6.  Nylon is a somewhat unusual polymer in that it readily absorbs water, and its properties change significantly when it does.  The material is hydrophilic enough that even atmospheric humidity causes significant water absorption.  The issue is not simply water between the fibers of the rope, making it heavy, or stiff when frozen, but a fundamental change in the material of the rope itself.  The glass transition point drops considerably, causing tensile strength to fall and creep to rise.  A wet nylon rope is weaker and less able to recover from stretching in just the same way it would be if heated excessively.
In applications involving water nylon should be avoided when possible and that is exactly what one sees in caving and canyoneering ropes: the use of polyester, which is not significantly affected in this manner.  Unfortunately polyester does not have the elongation and energy absorption needed for a dynamic climbing rope so rope manufacturers are left trying to patch an inherent problem, and that patch is hydrophobic treatment as described in the other answers.


Cell phone questions in two separate countries are considered duplicates

Question: I posted a question about using cell phones in Japan and now want to post the exact same question except being for South Korea. It is regarded as a duplicate. So how do I go about getting answers to these two things? Should I put it in one question or try to reformulate?
Comment: Regarded as a duplicate by the site or by some user or moderator? What exactly are you seeing when you try to post this question?
Comment: One thing, be careful you didn't accidentally paste the exact same question title in with the word "Japan" because a new site feature recently is banning questions with exactly the same wording. If you replaced "Japan" with "Korea" it should be fine.
Comment: Won't be a moderator thing as we'd have to manually close it.  Must be automated.  Could you paste your description and title in here so we can see the problem ourselves?
Comment: I replaced "Japan" with "South Korea" - otherwise the text is the same. I also replaced the japan tag with korea. The text I get back (automatically) is: "Oops! Your question couldn't be submitted because: Sorry, this is a duplicate of Cell phones in Japan"
Comment: I want to ask it for you but I also want you to get the rep for it! Why don't you try asking it in completely new wording rather than copying and pasting? They probably do a check for percentage of exactly same text as an anti-spamming measure.
Comment: Ok - I'll try that. So you think that it should be 2 separate questions?
Answer: The aim of the duplicate check is to avoid a large number of questions that are the same, and unfortunately it can't differentiate between true duplicates and questions that are semantically different but look similar.
It makes sense to me to ask these as two separate questions, so I'm glad you were able to get this working by rephrasing one of them.
Answer: Yes, this puzzles me too. I don't understand the fear for a multitude of duplicates. Modern Internet technology (google) enables easy organization of these so-called duplicates. As a person living in one country and working in another I can confirm that asking about phones in one country are not duplicate to the other. Maybe there should be a button: "Ask the same question for country Z". ;)


RGB LED strip 100m

Question:
Hello all,
I'm working on RGB LED strip SMD 5050 5m
This all setup is working on 12v 2A with about diagram.
I want to use same connection with 100m of LED strip using Arduino.
Please support regarding this topic.
H/W details :
Arduino uno
Hc-05 bt module
12v 1.2 amp supply
Td140 pnp transistor
SMD 5050 LED strip 60 LED/m
Comment: Where do you want the 100m of wire? Between the PSU and the LEDs? Between the Arduino and the transistors? Between the transistors and the LEDs? Between the Arduino and the bluetooth transceiver?
Comment: I want to use 100m LED strip
Comment: 100m of 60LED/m? i.e. 6000 RGB LEDs or 18000 LEDs?
Comment: You cannot drive 100m of LED strip from one side. That would be 200A. Even at only 5m I needed to provide the power at both sites to prevent the strip from getting too hot. So you would have to provide power also every few meters into the strip.
Comment: there's a limit to how many LEDs you can address w/WS2812, i don't recall, but it's far less than 6000
Answer: 100 Meters of 5050 SMD strip LEDs will require roughly 120-200 amps. However, you cannot drive a 100 meter strip from a single power supply. The thin copper traces in the LED strip can only handle so many amps before they become too hot (your LED strip would melt like ice cream on a hot summer day). This is why led strips are usually limited to 5 meters (~5A). You would need separated strips and power supplies for the 100 meters worth.
You have the correct idea with your circuit for turning the leds on/off (in fact the wiring is correct) but the transistors you plan on using are the wrong type. To switch the LEDs to ground you would need a low side switch, which would be a NPN transistor. Be careful however, transistors typically switch currents less than 1 amp. For higher power applications I would suggest using a N-channel Mosfet. A quick search on Sparkfun turns up the FQP30N06L, which is capable of handling 30A (with a heatsink, probably less than half without), and is also logic-level, meaning it can be driven with a 5 volt source (Arduino I/O pin).
Keep in mind, whatever current your LED strip draws, the ground switch (NPN transistor, N-Channel MOSFET, relay) must be able to handle the same current.
Good luck with your circuit!

RGB LED strip 100m


How do I install Samba 4?

Question: Does anyone have a good installation manual to install samba 4 on ubuntu 10.10.
I found a lot of articles to install samba 4 but all are different and most will not work with my ubuntu version. So some questions has been arised like "Where get I the recent built for ubuntu" and so on.
Answer: There is a samba 4 package in Ubuntu's Universe repository. It's currently in the alpha phase, so it's not recommended for critical applications.
You can install it by searching for "samba4" in the Software Center or Synaptic or by typing sudo apt-get install samba4 in a terminal.
Answer: I heard about a daily built on www.launchpad.net which can be used in non productiv environment. The samba4 package from ubuntu is an old built - as I heard.
But there is no complete guide to install and set up an active directory in samba4.
Comment: There is a PPA with daily built packages here: http://launchpad.net/~samba-team/+archive/daily
Answer: check out https://wiki.samba.org/index.php/Samba4/HOWTO. I just installed it on debian after this manual and so far i can connect windows xp and dabian (ther is no linux client howto, but i used a linux-active directory howto and it worked).
To set this up do NOT install kerberos or ldap, it is shipped with the installation in this howto. But you do need to install bind yourself and do the changes described.
Answer: I would not recommend installing samba4 yet as it is an alpha version if you don't have a very good knowledge of samba or a very specific reason not to install samba3... and if so you should know very well how to install and configure it or things will not work very well in your network very soon.

How do I install Samba 4?


induction of van der Waals forces

Question: Is there a way to maximize van der Waals forces between two materials, by electrifying one material, thus creating dipoles/instantaneous dipoles and thus creating stronger van der Waals forces? has this been documented?
Comment: I think you shoud have read more about van der Waals forces and electrifying objects. What are you really asking about?
Answer: Perhaps an example of what you mean can be seen with the noble gases. As you go from helium to neon to argon, etc. you are adding inner electron shells and making the electron cloud around each atom more polarizable. That in turn increases the van der Waals attraction between atoms, resulting in a dramatic increase in boiling points down the series.
You might be able to make an atom more polarizable (and increase van der Waals forces) by electronically exciting it to a higher energy level. Can anyone else elaborate on that possibility?

induction of van der Waals forces


Can a player swap more than 4 resources of the same kind with another player

Question: In one of the games I played recently, I was finally in competition with another player, the rest of the 2 players effectively having conceded. A 3rd player who I had a rivalry with swapped 8 lumbers for 1 sheep from the player I was in competition with, effectively helping that player win and finish the game in that turn itself. Is this kind of trade allowed according to the rules? I am sure it is something to be frowned upon, but I still wonder if it is possible to make the trade legally.
Comment: There's very little that you can do about *kingmaking* in Catan, so I would suggest playing a different game (or playing Catan with different people if that is an option).
Comment: Related: https://boardgames.stackexchange.com/questions/7487/is-kingmaking-in-multiplayer-games-a-problem-that-can-be-fixed
Comment: Would you consider it something to be frowned upon if the trade allowed your competitor to take longest road off you and not win, thus allowing the 3rd-place player a better chance of coming back?
Answer: There is nothing in the rules that prohibits this move.
The following excerpts are taken from the 5th edition rules.
Page 7, "Domestic Trade":

On your turn, you may trade resources with the other players (after rolling for resource production).  You and the other players negotiate the terms of your trades - such as which cards will be exchanged.  You may trade as many times as you can, using single or multiple cards.  However, you cannot give away cards, or trade matching resources ("trade" 3 or for 1 ore, for example).

Page 14, "Trade":

After you roll for resource production, you may trade with other players (domestic trade) or with the bank (maritime trade).

You may trade as long as you have resource cards.
If you decide not to trade during your turn, no one can trade.
You may trade with another player between your turns, but only if it is his turn and he elects to trade with you.
You may not trade with the bank during another player's turn.
You may not give away cards.
You may not trade development cards.
You may not trade like resources (e.g., 2 wool for 1 wool).

There is no mention of restrictions on the number of cards that can be traded in a single move.


How to setup ssh only for Tor?

Question: On my VPS server, I would like to set up ssh access only from the Tor network. I know that if I torrify ssh it will be visible only on the Tor network. However, my server works only in clearn and it is not configured Tor. Is there a possibility to configure ssh so as to permit a login from a specific network, and that network will be Tor? Can I define the whole Tor network as the subnet?
Answer: To define the whole network as subnet, logically, first you would need to know all tor nodes/servers. there are several thousands of nodes and some of them are not public. but if you can collect all that information, maybe it would be possible. but as I know, many tor nodes are at amazon hosting, I think if they just restart server they can change IP address.
Comment: yeah, i know i am a dreamer :) but thanks for answer
Answer: Below just my though, PollyPocket.
You could define firewall rules to drop all incoming packets to your sshd port except those originating from defined tor exit nodes. Then you could use these exit nodes only to connect to your sshd.
The issue here is a lack of vhost support by sshd so you cannot be sure whether an incoming connection originates from tor or an exit node itself (in case an exit node acts as a clearnet proxy too) as long as you don't control the exit node yourself. A workaround would be to allow only these exit nodes hosted securely by you.
Interesting question very much. Hope to see further discussion.
Comment: You may be also interested in mktorfw.sh script of [tor-hacking-utils](http://pseudo-flaw.net/resource/tor/hacking/README) from pseudo-flaw.net. It _"create a very restrictive iptables firewall rules that allow only those IP/port combinations found in /var/lib/tor/cached-descriptors* (or /var/lib/tor/cached-routers* for pre-0.2.0.7)"_
Comment: Cool, thanks, sound interesting. I am using private bridge and own private exit node :)

How to setup ssh only for Tor?


What is the temperature at steering rack?

Question: What is the average temperature of air around steering rack? Can it be greater than 70  C ? Actually I have to put a sensor over there whose threshold temperature of working is 75  C.
Comment: Are you testing the temp of the rack itself, or just the ambient temp where rack is located?
Comment: ambient temperature of air around rack.
Comment: Considering a steering rack's position in a vehicle (low mounted, below the engine, behind the radiator, in open air), I would bet this area is only going to be slightly warmer than ambient air temperature. There will be some air flow from the radiator hitting this area, but it will be mixed from air coming from under the car. I would say, you could try your temperature sensor and doubt you're going to see temp above the 70C you are asking about. It would have to be a hot day on hot pavement to exceed this in my approximation.
Answer: I don't think you ought to be designing for average temperature--I think you're more looking for a peak temperature under expected conditions for your application.
Depending on atmospheric and driving conditions, the temperature could easily exceed 75 C (stuck in traffic in the summer, for example).
Many types of automotive wiring are rated to 80-125 C, you might use this as a reference point for suitability.
Answer: You could probably measure the temperature of the power steering fluid reservoir as a ballpark idea; that fluid is pumped through the rack and likely keeps it within 10deg.


Looking for linux distro

Question: I'm looking for linux distro capable of natively running windows apps with .net, DirectX and all other stuff. Does such windows replacement even exist ?
Comment: no... Windows applications are written completely differently to UNIX-based systems. What you should do is install a simple, clean distro (my favourite being mint) and install WINEHQ
Comment: @treyBake Actually, BackSlash Linux can but it is discontinued. I found ReactOS but it is not really linux, or complete. It is obvious goal to achieve. Majority of applications runs on windows so why don't support them too, right ? Windows API is public so you just have to create system what is able to correctly respond to API calls. Well, said easier than done.
Comment: That's not the problem, the problem isn't Linux supporting Windows. It's Windows-based developers supporting UNIX. They use different binaries and file types to do the same thing. Most Linux distros have access to Wine, Virtualbox and other emulator-type programs to run Windows-based software. This is really all you need rather than a complete OS to handle running Windows, if you need to run more Windows programs than Linux, then just use Windows
Comment: Dual boot is what I do.  I share my home partition with both OS's.  This provided me with a way to transition from Win to Linux on my laptop.  Yes, a reboot is required to switch OS's.  And file sharing has to be done in NTFS, which has its limitations in Linux.
Answer: No, there's no Linux that can natively run [any] Windows apps with .net, DirectX and all other stuff.
There are a couple options that might come close:

Run any Linux, and:

Run some compatible windows programs with WINE
Run a virtual machine (VM) program (hypervisor) like VirtualBox, and install Windows inside a VM, then run almost any program in the virtual Windows.

Run Windows 10, and run any Windows programs, but also "install linux inside Windows," with the Windows Subsystem for Linux (WSL).
Try searching for the "Windows subsystem for Linux"
Wikipedia says:

WSL provides a Linux-compatible kernel interface developed by Microsoft (containing no Linux kernel code), which can then run a GNU user space on top of it, such as that of Ubuntu, openSUSE, SUSE Linux Enterprise Server, Debian and Kali Linux. Such a user space might contain a Bash shell and command language, with native GNU/Linux command-line tools (sed, awk, etc.), programming language interpreters (Ruby, Python, etc.), and even graphical applications (using a X11 server at the host side).
Limitations
This subsystem is not capable of running all Linux software, such as 32-bit binaries, or those that require specific Linux kernel services not implemented in WSL. Since there is no "real" Linux kernel in WSL, kernel modules, such as device drivers, can't be run.
It is possible to run some graphical (GUI) applications (such as Mozilla Firefox) by installing an X11 server within the Windows (host) environment (such as VcXsrv or Xming), although not without caveats, such as the lack of audio support or hardware acceleration (resulting in poor graphics performance). Support for OpenCL and CUDA is also not being implemented currently, although planned for future releases.
That said, Microsoft explicitly states that WSL is oriented to the development of applications, and not for desktop environment or production servers, recommending the use of virtual machines (Hyper-V or Kubernetes) and Azure for those purposes.

Looking for linux distro


Can Ghidra do backward symbolic execution?

Question: I am searching for a tool which can do backwards symbolic execution on x86 binaries. More specifically, I would like to give the tool a program state(PC value + registers + memory content) and have it give me a set of inputs to the program that would result in that state. Can Ghidra do this? Or does anyone know if there are any other tools which can?
Comment: I think what you want is rather forward symbolic execution. Angr can do it. You would execute the program from starting state to the goal state while marking the inputs as symbolic. You can add the constraints to the goal state such as the value of this register must be equal to this value.
Answer: Looks like you also asked the question over on the Ghidra GitHub page and the answer there is the same.
Essentially, no, Ghidra does not do that. But you should investigate Angr, Klee, Bitblaze, or others.


What do TCP/UDP add to "raw ip"?

Question: I know that both TCP and UDP are built on top of IP, and I know the differences between TCP & UDP, but I'm confused about what exactly "raw ip" is. Would it be fair to say that TCP & UDP both implement IP, but that IP in and of itself isn't capable of transferring data? Or is IP some very low level form of communication, which is further abstracted by TCP and UDP?
Comment: IP certainly does transfer data - it carries those TCP/UDP packets, for example. It's not usually application-usable, though. UDP is a very raw protocol on top of IP, but it already adds *port numbers*, allowing multiple services to handle UDP traffic concurrently on a system. Otherwise, you'd only have as many services as you have IP addresses.
Comment: To further some of the comments below (not enough for a full answer, just for the sake of clarity): IP is a protocol that transports data. TCP or UDP are protocols that transport data too. TCP and UDP often sit on top of IP so for IP it's data payload can be TCP or UDP. Ethernet is often used to carry IP. So for example an Ethernet frame may carry and IP packet as it's data payload and that IP packet maybe carrying a TCP segment as it's payload, and so on. You end up with a stack of protocols like this https://goo.gl/1uEYtC
Answer: IP is a Layer 3 protocol. TCP/UDP are Layer 4 protocols. They each serve different purposes.
Layer 3 is in charge of end to end delivery. Its sole function is adding whatever is necessary to a packet to get a packet from one host to another.
Layer 4 is in charge of service-to-service delivery. Its sole function is to segregate data streams. Your computer can have multiple programs running, each which sends/receives bits on to the wire. IE: You could have multiple browser tabs running, streaming internet radio, running a download, running some legal torrents, using a chat application, etc. All of these receive 1s and 0s from the wire, and Layer 4 segregates each data streams to the unique application that needs them. Here is an illustration:

IP is unable to deliver a packet to the correct service/application. And TCP/UDP is unable to deliver a packet from one end of the internet to the other.
Both TCP and IP work together to enable them both to achieve the "end-goal" of Internet communication.
Data that needs to get from one host to another is generated by the upper layers of the OSI model.
This data is passed down to L4 which will add the information necessary to deliver the data from service to service, like a TCP header with a Source and Destination Port. The Data and the L4 header is now referred to as a segment.
Then the Segment will be passed to L3 which will add the information necessary to deliver the segment from end to end, like an IP header with a Source an Destination IP address. The L3 header and the segment can now be referred to as a Packet.
This process is known as Encapsulation and De-encapsulation (or sometimes decapsulation). Here is an animation of how it works:

If this doesn't make sense, I suggest reading more about the OSI model, and how each layer has different responsibilities that all work together to accomplish moving a packet across the Internet.
Comment: Note that the Internet only loosely follows the OSI model.
Comment: `inside an IP header is usually a TCP or UDP header` is not correct, the TCP/UDP header is not inside the IP header, it's inside the data portion of the IP packet.
Comment: "Its sole function is to segregate data streams", while that is mostly true for UDP (there is also a checksum but meh) it is certainly not true for TCP.
Comment: @immibis True, but I have yet to see where following the OSI model has lead to a gaping implementation or understanding travesty. For the most part, and especially for someone newly approaching Internet technologies, its more valuable to continue thinking along the constrains of the OSI model.
Comment: @Eborbob I can see how its wrong depending on how look at the sentence. Honestly, that whole paragraph could use a reword, I'll try to get to it later tonight. Thanks for pointing it out.
Comment: @PeterGreen Yes, TCP does *more* than *just* segregate data streams. But I intentionally avoided going into the details of UDP and the details of TCP and wanted just to focus on the goal of L4 as a layer.
Comment: @Eddie How about the massive confusion between the top 3 layers? What is the presentation layer in HTTPS?
Comment: @immibis Fair point. Do you have any examples at L1-L4?. Most *Network* Engineering tends to focus just on those four layers.
Comment: @Eddie I've heard that TCP's graceful close is considered to be a layer 5 feature.
Comment: @immibis Odd. I have never heard that. Moreover, I can't see a negative effect of considering it a L4 (if it isn't already).
Answer: IP can transmit data just fine, the problem is what happens when that data gets to the other end. The only identification information is the IP addresses of the hosts and a protocol number. Neither of which provide any way to distinguish what socket the data is for.
It is possible for programs to use IP directly through what are known as "raw sockets" but the lack of a proper mechanism for deciding which socket to send data to raises security and performance concerns (the kernel has to send the data to all raw sockets for a given protocol number). For this reason the ability to open "raw sockets" is usually restricted to root (or your platform's equivilent).
UDP is a fairly minimal layer over IP. It adds port numbers to identify what socket the data is for and a checksum (to reject corrupt packets). The application remains responsible for dealing with lost packets, controlling congestion and splitting the data into appropriate sized packets.
TCP is a more complex protocol that in addition to providing port numbers and checksums splits byte-streams into packets, reassembles then at their destination and provides congestion control and recovery features.
Comment: Of all the articles and writings on this subject on the internets, this answer distills everything down to the truth that matters.
Answer: IP is an OSI layer-3 protocol, while TCP and UDP are OSI layer-4 protocols. As a layer-3 protocol, IP can carry many different layer-4 protocols. TCP and UDP are probably the most common, but they are not the only ones. Layer-4 protocols are what applications use as their end-to-end connections. IP transports layer-4 protocols from network to network (host-to-host). Layer-2 protocols, like ethernet, transport layer-3 protocols on a LAN.
You should research the OSI model, but realize that it is a conceptual model, and the real world often doesn't exactly match.

What do TCP/UDP add to "raw ip"?