Add Multipath TCP (MPTCP) Support to the Java Networking API

[Alan Bateman]

On 18/09/2025 08:36, Geliang Tang wrote:
> I have completed the implementation. Thanks to Xiang Gao and Gang Yan
> for their help. I added a new option named TCP_MPTCPIFY in
> jdk.net.ExtendedSocketOptions (the name was chosen to align with
> 'mptcpize' tool in mptcpd and 'mptcpify' in BCC). When this option is
> set, it calls the JNI function mptcpify0.
>
> Following your suggestion, mptcpify0 is implemented by referencing
> Java_sun_net_sdp_SdpSupport_convert0. It creates a new MPTCP socket,
> uses dup2 to duplicate it, and then closes the redundant socket.
>
> I have added your tag in the patch:
>
> Suggested-by: Alan Bateman<alan.bateman at oracle.com>
>
> I also included a test example for this option. All modifications can
> be found here:
>
> https://github.com/openjdk/jdk/compare/master...geliangtang:jdk:master
>
> Please provide any feedback or suggestions. As for the next steps,
> should I create a new issue for this feature and submit a proper pull
> request?

Thanks for confirming that the direction suggested works. This at least 
demonstrates a feasible direction that does not impact the standard API.

As regards next steps then I think start a thread on net-dev to try to 
get input. I think it would be useful to show potential scenarios where 
it would be useful and what performance benefits might come from it. I 
think also useful to talk about whether this is something that an 
application has to opt into, maybe there are other configuration 
approaches that would not require application changes?

I think it would be useful to discuss testing. If this feature were 
integrated into the JDK then how would it be tested? Who would test and 
maintain it? There is a JDK release every 6 months, is someone going 
test it with each release?

If it goes ahead then there the name of the extended socket option and 
its API docs will need discussion. Details around state will need to be 
specified, would it be supported to set TCP_MPTCP when bound or 
connected, does it make sense to ever attempt to set it to false after 
setting it to true (these are just examples of the details that will 
need to be specified, they are not important right now).

-Alan.
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