Add Extended MPTCP (Multipath TCP) Socket Option Support to OpenJDK

[Alan Bateman]

On 23/09/2025 17:03, Jaikiran Pai wrote:
> :
>
> The bigger question, I think, is whether constructing a "socket()" 
> with a specific "protocol" value in this manner by trying to adjust 
> the implementation in the code which deals with socket options is the 
> right thing to do. I realize that this proposal of prototyping it as a 
> socket option was suggested in response to your first email, so I am 
> not saying this direction should be abandoned, I just haven't fully 
> grasped if this can cause us problems with this approach.

Just to point that using dup2 to "convert" the socket to IPPROTO_MPTCP 
is the same trick that the JDK did for SDP. I think the main question is 
whether it is compelling enough for the JDK to include, and if so, how 
would it be tested and maintained.

I'm not too concerned about the API docs right now. As with all "socket 
options", there are many details, esp. around state, that would need to 
be specified. The JDK has several examples of options that don't support 
both "set" and "get".

Another direction that would allow the existing APIs to be used if to 
expose a restricted method to get at the raw file descriptor, something 
we've toyed with several times. That approach would allow MPTCP to be 
implemented as its own library, wouldn't need anything in the JDK, but 
would require users to run with a CLI option to enable native access 
(the current way to allow restricted methods).

-Alan.