[U-Boot] [PATCH] net: More BOOTP retry timeout improvements

[Thierry Reding]

On Tue, Aug 19, 2014 at 09:59:00AM -0600, Stephen Warren wrote:
> On 08/19/2014 02:06 AM, Thierry Reding wrote:
> >On Mon, Aug 18, 2014 at 01:01:17PM -0500, Joe Hershberger wrote:
> >>Hi Thierry,
> >>
> >>On Mon, Aug 18, 2014 at 1:45 AM, Thierry Reding <thierry.reding at gmail.com>
> >>wrote:
> >>>
> >>>From: Thierry Reding <treding at nvidia.com>
> >>>
> >>>It's not unusual for DHCP servers to take a couple hundred milliseconds
> >>>to respond to DHCP discover messages. One possible reason for the delay
> >>>can be that the server checks (typically using an ARP request) that the
> >>>IP it's about to hand out isn't in use yet. To make matters worse, some
> >>>servers may also queue up requests and process them sequentially, which
> >>>can cause excessively long delays if clients retry too fast.
> >>>
> >>>Commit f59be6e850b3 ("net: BOOTP retry timeout improvements") shortened
> >>>the retry timeouts significantly, but the BOOTP/DHCP implementation in
> >>>U-Boot doesn't handle that well because it will ignore incoming replies
> >>>to earlier requests. In one particular setup this increases the time it
> >>>takes to obtain a DHCP lease from 630 ms to 8313 ms.
> >>>
> >>>This commit attempts to fix this in two ways. First it increases the
> >>>initial retry timeout from 10 ms to 250 ms to give DHCP servers some
> >>>more time to respond. At the same time a cache of outstanding DHCP
> >>>request IDs is kept so that the implementation will know to continue
> >>>transactions even after a retransmission of the DISCOVER message. The
> >>>maximum retry timeout is also increased from 1 second to 2 seconds. An
> >>>ID cache of size 4 will keep DHCP requests around for 8 seconds (once
> >>>the maximum retry timeout has been reached) before dropping them. This
> >>>should give servers plenty of time to respond. If it ever turns out
> >>>that this isn't enough, the size of the cache can easily be increased.
> >>>
> >>>With this commit the DHCP lease on the above-mentioned setup still takes
> >>>longer (1230 ms) than originally, but that's an acceptable compromise to
> >>>improve DHCP lease acquisition time for a broader range of setups.
> >>>
> >>>To make it easier to benchmark DHCP in the future, this commit also adds
> >>>the time it took to obtain a lease to the final "DHCP client bound to
> >>>address x.x.x.x" message.
> 
> >>>  void BootpReset(void)
> >>>  {
> >>>+       bootp_num_ids = 0;
> >>>         BootpTry = 0;
> >>>         bootp_start = get_timer(0);
> >>>-       bootp_timeout = 10;
> >>>+       bootp_timeout = 250;
> >>
> >>What is the impact on the time it takes for you to get an address assigned
> >>in your environment with the rest of this patch, but without this change of
> >>the initial timeout?  It seems like it shouldn't impact you negatively but
> >>will still help Stephen's original case.
> >
> >Leaving the initial timeout at 10 ms increases the time to get a lease
> >from 1230 ms to 5350 ms. I suspect the reason for that is that the DHCP
> >server will queue an ARP ping for each request and process them
> >sequentially. Each of those takes about 600 ms and I see U-Boot sending
> >out a total of 9 broadcasts. So that's 5400 ms delay only due to the
> >requests that haven't been answered. The 8th broadcast is probably the
> >one that U-Boot receives a response for, hence explaining the 5350 ms.
> >
> >Even the 250 ms timeout will make things a lot better for Stephen's use
> >case. Given the earlier discussion it seems like the DHCP server in his
> >network doesn't check for existing users of an IP address using ARP
> >pings and replies fairly quickly, so I suspect the whole process to take
> >around 300 ms for him. That's still a lot better than the 1230 ms that
> >it takes on my setup (which used to be somewhere around 630 ms before
> >Stephen's original patch).
> 
> I'm pretty sure I have seen ARP pings, although I guess the timeout on my
> DHCP server must be much more reasonable, since the whole DHCP process
> completes for me on the first request U-Boot sends.

Are you sure they are ARP pings from the server? I've seen DHCP clients
using ARP pings to check their IP address before using it. Something on
the order of 20 or 40 ms for an ARP ping timeout would be surprising.

Thierry
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