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Rapid DHCP: Or, how do Macs get on the network so fast?

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Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#121

Post by rektide »

saurik wrote: post #15

> This network recognition technique allows the Mac to very rapidly discover if it is connected to a known network. If the network is recognized (and presumably if the Mac knows that the DHCP lease is still active), it immediately and presumptuously configures its IP interface with the address it knows is good for this network. Ok, seriously? That isn't a bug in an implementation somewhere, but in fact a feature that…

The Mac's DHCP client is using the router's mac addresses to verify it's on the right network, and it already knows it still holds the lease. What the fuck more do you want? What are you afraid of?

The only conceivable grief you can put forward is that the ARP check isn't good enough. The client already owns the lease. If there's IP contention, it's because either a) it's not on the right network after all (arp check) or b) the server's fucked up and it's the servers fault (dhcp server allocated an IP twice).

Ok, so assuming the DHCP server isn't faulty may be a bit of a stretch. But coding your client to assume the server can do it's job does not deserve an "Ok, seriously?" hurf-burf post.

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#122

Post by jarek »

shinratdr wrote: post #120
jarek wrote: post #103

Earlier quoted context omitted.

You're right. I guess it's more comparable to walking up to a checkout or a cash register without looking to see if anyone is waiting in line. Great UX if you don't look around you.

It's really not. There is no real world comparable act, frankly. It is what it is, and what it is is not a big deal.

I cannot emphasize enough how much I disagree with the idea that potentially inconveniencing other network users for a small gain of your own is not a big deal.

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#123

Post by saurik »

rektide wrote: post #121
saurik wrote: post #15

> This network recognition technique allows the Mac to very rapidly discover if it is connected to a known network. If the network is recognized (and presumably if the Mac knows that the DHCP lease is still active), it immediately and presumptuously configures its IP interface with the address it knows is good for this network. Ok, seriously? That isn't a bug in an implementation somewhere, but in fact a feature that…

The Mac's DHCP client is using the router's mac addresses to verify it's on the right network, and it already knows it still holds the lease. What the fuck more do you want? What are you afraid of? The only conceivable grief you can put forward is that the ARP check isn't good enough. The client already owns the lease. If there's IP contention, it's because either a) it's not on the right network after all (arp check…

I take it you have not read any of the rest of this thread?... all of the comments about how DHCP in the wild doesn't work that perfectly, or people saying "this happens all the time with my / my wife/friend/boss's Mac"?

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#124

Post by [deleted] »

saurik wrote: post #70
bradleyland wrote: post #58

Earlier quoted context omitted.

The DHCP stack in iOS does some profiling that should reliably detect whether it is, in fact, on the network that it expects. The most significant of which is the verification of the DHCP server's Ethernet address as correlated to IP. If you are on a known network, your DHCP lease is still active, and network equipment is working as expected, all of these assumptions are safe. The most common failure scenario is wher…

Down-thread of this comment, people have already discussed numerous issues with the assumption that DHCP leases are stable when not in use (such as that servers often choose to allocate out of an artificially small pool of addresses, and ping old addresses to determine if they are in use before simply reclaiming and reassigning them).

[deleted]

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#125

Post by rektide »

jarek wrote: post #85
sedev wrote: post #82

Earlier quoted context omitted.

I think that you're wrong because, basically, if that 'steal an IP address' scenario happens, it means that the DHCP server has in some way broken its promises. That _happens_ in production environments, but I'd much rather clients use behavior like this, that assumes that a DHCP server will keep its promises about things like lease length, than assume the worst about the DHCP server. The clients should first assume…

So here's my understanding of the situation: when a DHCP server runs out of leases to assign, it will kill the oldest (in terms of use) lease. If the device that held the lease is not on, for relevant values of "on", at the time, it will not be notified about this. The usual (perhaps standard, I'm not sure) process has the device confirming its DHCP lease when coming back on, and in this situation the device would be…

Please name one or more DHCP server which will knowingly violate a lease it has granted, via "kill the oldest" or any other means.

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#126

Post by vacri »

flogic wrote: post #79

Rather than asking why the mac is so fast, the correct question is "why the hell is dhcpcd so slow?". There's a full second before it does anything.

I think it's apples and oranges to some degree - a macbook pro with a venerable operating system versus a tablet with one that's still heavily in development.

Better to compare galaxy vs ipad tablets, or osx vs other unix vs windows

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#127

Post by [deleted] »

shinratdr wrote: post #119
sid0 wrote: post #94

Earlier quoted context omitted.

And yet it is exactly as dickish and disrespectful of others. Just like Apple as a whole, it seems.

Meh. Treating developers and the back end gingerly results in no benefit for consumers, in fact by and large it results in in a pretty clear loss. Characterize it however you wish, as a user I'm going to go with the platform that focuses on pleasing me rather than making the lives of other users or developers easier.

[deleted]

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#128

Post by rektide »

tonfa wrote: post #24
pinko wrote: post #9

This is a great example of Apple's detail-oriented focus on real-world user experience, and helps explain why people prefer Macs even if they can't always explain why. Lots of little things just work better. You (where "you" == myself and many others, even if not /you/ personally) are left overall with an experience of less frustration.

Well, in that case they might not be following the protocol. And in some cases break the connectivity for others...

DHCP (rfc2131) doesn't really talk about IP exhaustion.

It's undefined behavior, and there's a bunch of people in here saying that server's have every right to violate DHCP leases, otherwise someone can take over a network by continually claiming all of a Class C by continually making DHCP requests.

That argument makes sense, but it goes very strongly against the grain of what I would think a DHCP lease meant, whcih is that it's a contract for a specific amount of time. If it is indeed a contract for a specific amount of time, the client has every right to claim what they're contractually obliged to. This was my initial assumption, and I believe it's Apple's rather valid assumption.

It's not a case of Apple not following the protocol. It's a case of the real world being more complex than the protocol.

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#129

Post by bradleyland »

th0ma5 wrote: post #108
__david__ wrote: post #105

Earlier quoted context omitted.

Which specification does it violate?

rfc 2131? seems to say the client can ask for an unlimited lease, but the server can specify a finite one. this article seems to be saying that this thing will use the last one leased no matter how long it's been off the network

The DHCP client acts as expected. The behavior observed by the post author occurs before DHCP negotiation executes. I know this seems like a technicality, but it's the truth. The sniffing and assumed use of an IP address occurs outside the DHCP request/response cycle. It's used ahead of time, and discarded if the DHCP 'request' request is denied.

The only people bent about this are people who have any idea what's going on behind the scenes. In the rare cases where this is a problem, the issue is quickly resolved. If this were a "real" problem, we'd have seen a lot more issues by now. The Apple discussion boards would be lit up with complaints. They're not. This is a non-issue. Only pedants are stuck on the fact that there's no RFC for it.

Re: Rapid DHCP: Or, how do Macs get on the network so fast?

#130

Post by rlpb »

jarek wrote: post #113
rlpb wrote: post #107

Earlier quoted context omitted.

The problem is that the DHCP server re-issued a valid lease to a second computer. Surely this is an issue with the DHCP server not playing nice, rather than Apple?

Do you feel there is a better, more tolerant, and less error prone course of action the DHCP server could take if it runs out of leases?

If we are able to modify the DHCP server to do something sensible, why not modify default router settings to issue /16 addresses instead of /24? Or even 10/8? There's plenty of RFC1918 space to not run out.

NAT state tables won't have to stretch that far as they can time out unused connections just like they do already.

Would there be any unintended side-effects with doing this?

If you don't like the NAT, go ahead and issue IPv6 :-)

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