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

[Geliang Tang]

Hi Alan,

Thanks for your reply.

On Fri, 2025-09-05 at 10:28 +0100, Alan Bateman wrote:
>  
> On 05/09/2025 09:43, Geliang Tang wrote:
>  
>  
> >  
> > :
> > 
> > 3. Proposed Java API Changes
> > 
> > The goal is to allow Java applications to opt-in to using MPTCP
> > when
> > creating sockets, without breaking existing code. The proposed
> > changes
> > are additive and backward-compatible.
> > 
> > The core idea is to add a boolean mptcp parameter through the API
> > layers, from the public Socket class down to the native system
> > call.
> >  
>  
>  (moving the discussion to nio-dev and net-dev).
>  
>  Adding new constructors to legacy Socket/ServerSocket may work for
> your initial prototype but would be problematic to propose as it
> would bake non-standard features into the standard API. Also many
> high performance servers use SocketChannel rather than Socket.
>  
>  One suggestion for a next prototype is to model the enablement of
> MultiPath TCP as as a JDK-specific socket option (see
> jdk.net.ExtendingSocketOptions). All of the standard APIs for
> networking sockets define a setOption method for setting socket

I was initially concerned that calling setOption() after Socket()
creation to convert TCP to MPTCP was too late. Creating an MPTCP socket
must be done during the socket() system call.

> options. If modeled as a socket option then enabling can create a new
> AF_INET6/SOCK_STREAM/IPPROTO_MPTCP socket and then dup2 it into place

Until I saw here, it should be feasible if the socket can be created
again and dup when calling setOption().

I will try to reimplement MPTCP support using
jdk.net.ExtendingSocketOptions soon and give you feedback.

Thanks,
-Geliang

> so that the original AF_INET6/SOCK_STREAM/0 socket is closed.
> Enabling can be made to fail if the socket is already bound. It could
> copy over existing socket options if needed. Look at the built-in
> (and no longer used) SDP support for an example that does similar
> with AF_INET_SDP/SOCK_STREAM/0. The only API surface would be a
> socket option defined in jdk.net.ExtendingSocketOptions.
>  
>  -Alan
>  
>