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

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354 points timdoug | 3 comments | 12 Jul 11 17:04 UTC | HN request time: 0.007s | source
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saurik ◴[12 Jul 11 17:52 UTC] No.2755677[source]▶
> 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 Apple actually is proud of? Am I the only one who finds that if you get a room full of people sitting around with Macs at least one person gets their IP address stolen by someone else?

(edit: I just got downvoted, and then asked the people in the room with me, and they seemed to agree with my perceived correlation regarding the "another computer is using 192.1.0.1" issue... instead of just downvoting, maybe reply? It is actually quite common that DHCP leases on a network get reset for various reasons, and if you just jump on the network without revalidating your lease, you are actually quite likely to just "presumptuously" steal someone else's IP address.)

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bradleyland ◴[12 Jul 11 20:15 UTC] No.2756333[source]▶
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 where the DHCP server has lost its DHCP lease table. For many consumer routers, this will occur when the router is rebooted. If you have to reboot your router frequently, you should replace it. Even if you do encounter a failure mode, the error will clear a second or two later when a new address is negotiated via DHCP and the subsequent ARP update.

This is very clearly a trade-off. Devices like the iPhone and iPad are switched on and off very frequently. The difference between 10s to network ready and 0.3s to network ready is more than significant; it's monumental. This is especially true for devices whose use-pattern will involve frequent on-off usage.

If you're uncomfortable with this trade off, you should stay away from Apple devices. Personally, I'll take the 10s.

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saurik ◴[12 Jul 11 20:52 UTC] No.2756511[source]▶
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).
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bradleyland ◴[12 Jul 11 21:01 UTC] No.2756557[source]▶
Like I said, if you're uncomfortable with the trade-off, avoid Apple devices. Personally, I only see the number of devices like the iPad/iPhone increasing, and I'm relatively confident that your normal user doesn't give a damn about any of this. They want the device to work when they turn it on.

I'd also argue that any DHCP server that applies that policy of artificially limiting the IP pool by re-issuing non-expired leases is an asinine implementation.

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1. jarek ◴[12 Jul 11 21:28 UTC] No.2756675[source]▶
Well, sure, but the problem is that the Apple device user doesn't suffer from the negative consequences of the non-standard action. Other users do.

Of course people are going to like devices that allow them to cut in line if there is a little bit of space.

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2. bronson ◴[13 Jul 11 15:12 UTC] No.2759193[source]▶
Non-standard action? It's standardized by RFC 4436!
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3. philjr ◴[14 Jul 11 23:17 UTC] No.2765597[source]▶
Not all RFCs are standards. See http://tools.ietf.org/html/rfc1796 entitled "Not all RFCs are standards" :)