RFR: 8284890: Support for Do not fragment IP socket options [v2]

[Michael McMahon]

On Fri, 15 Apr 2022 10:04:48 GMT, Daniel Jeliński <djelinski at openjdk.org> wrote:

>> Michael McMahon has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   builds in github action now
>
> src/jdk.net/windows/native/libextnet/WindowsSocketOptions.c line 73:
> 
>> 71:     if (family == AF_INET) {
>> 72:         opt = optval;
>> 73:         rv = setsockopt(fd, IPPROTO_IP, IP_DONTFRAGMENT, (char *)&opt, sizeof(int));
> 
> Why do we only use `IPV6_MTU_DISCOVER` but not `IP_MTU_DISCOVER`? As far as I can tell, `IP_DONTFRAGMENT` alone doesn't guarantee that the DF bit will be set.

I did (manually) check that the DF bit is set, though unfortunately, there's no straightforward way to test that in the regression test. We could have the same construction for AF_INET as AF_INET6 and try IP_MTU_DISCOVER first which won't work pre Windows 10/2019. So, I'll make that change.

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PR: https://git.openjdk.java.net/jdk/pull/8245