On 08/18/2014 12:45 AM, Thierry Reding wrote:
From: Thierry Reding treding@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.
Tested-by: Stephen Warren swarren@nvidia.com
So long as it's considered safe to accept DHCP responses to older requests, this approach seems fine. Having seen this patch now (I hadn't when I responded to the other thread), I guess I don't have a strong an opinion re: reverting my original patch vs. taking this; net maintainers, feel free to decide:-)