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

[Alan Bateman]

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 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 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
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