On Wed, 3 Aug 2022 09:43:44 GMT, Daniel Jeliński <djelin...@openjdk.org> wrote:
>> This patch partially fixes the issue where IPv6 sockets were fragmenting >> outgoing IPv4 datagrams even when IP_DONTFRAGMENT flag was set. >> Specifically, it fixes the issue on Linux and Windows. As far as I could >> tell, the issue is unfixable on Mac OS X. >> >> All systems have a separate DONTFRAGMENT flag for IPv4 and IPv6. Each flag >> only affects packets from its address family; if we want to disable >> fragmentation of both IPv4 and IPv6 packets sent by an IPv6 socket, we need >> to set both IPv4 and IPv6 flags. This is similar to other already existing >> options like IP_TOS or IP_MULTICAST_*. >> >> On Mac OS X it's impossible to set an IPv4 socket option on an IPv6 socket; >> attempting to do so results in an error. This is a known issue with Mac OS >> X; on that system we return false from >> [Net#shouldSetBothIPv4AndIPv6Options](https://github.com/openjdk/jdk/blob/2342684f2cd91a2e5f43dd271e95836aa78e7d0a/src/java.base/unix/native/libnio/ch/Net.c#L159) >> to avoid setting IPv4 options. >> >> As far as I can tell, non-privileged users have no way to check if the DF >> flag was set or if the packet was fragmented. I implemented a test that >> attempted to send a large packet over a physical interface and expected >> SocketException / EMSGSIZE; the test frequently failed for unrelated >> reasons, so I decided against including it. Loopback interface has infinite >> MTU on some systems, so can not be used for this test. >> >> Testing performed (with IP_DONTFRAGMENT flag): >> Windows 10: >> With this patch, IPv4 packets sent from IPv6 socket have the DF flag; >> sending an IPv4 packet larger than the interface MTU results in EMSGSIZE and >> no packet is sent. Without this patch, the packet is fragmented and sent >> without DF flag. >> Sending IPv6 packets larger than the interface MTU usually results in >> EMSGSIZE. It may succeed if the destination address is non-routable. >> >> For other systems I could not capture packets, so I can only report the >> observed behavior of sending packets. >> Windows 2012 and 2016 (IP_MTU_DISCOVER not supported): >> With this patch, sending any packet exceeding MTU size fails with EMSGSIZE. >> Without this patch sending a large IPv6 packet succeeds. >> If a packet is sent to a non-routable address, send succeeds, no error is >> reported. >> >> Linux: >> With this patch, sending any packet exceeding MTU size fails with EMSGSIZE. >> Without this patch, sending large IPv4 packets from IPv6 sockets succeeds. >> >> Mac OS X 12: >> Sending an IPv6 packet exceeding MTU size fails with EMSGSIZE. Sending large >> IPv4 packets from IPv4 sockets also fails with EMSGSIZE. Sending large IPv4 >> packets from IPv6 sockets succeeds. The patch does not change the observed >> behavior. > > Daniel Jeliński has updated the pull request incrementally with one > additional commit since the last revision: > > Revert Net.c comment changes The change is fine. The Net.c comment is not worth arguing over. ------------- Marked as reviewed by michaelm (Reviewer). PR: https://git.openjdk.org/jdk/pull/9575
