Implement lean "system" TCP/IP stack backend for TUN inbound

Co-authored-by: RPRX <63339210+RPRX@users.noreply.github.com>

proxy/tun/stack.go

@@ -1,7 +1,10 @@
 package tun
 
 import (
+	"context"
 	"time"
+
+	"github.com/xtls/xray-core/common/errors"
 )
 
 // Stack interface implement ip protocol stack, bridging raw network packets and data streams
@@ -13,5 +16,39 @@ type Stack interface {
 // StackOptions for the stack implementation
 type StackOptions struct {
 	Tun         Tun
+	MTU         uint32
 	IdleTimeout time.Duration
+	// Backend selects the concrete Stack implementation, see NewStack.
+	Backend string
+}
+
+const (
+	// StackGVisor selects the full-featured gVisor based stack (default).
+	StackGVisor = "gvisor"
+	// StackSystem selects the lightweight, Xray-native stack, see newSystemStack.
+	StackSystem = "system"
+)
+
+// NewStack builds the ip stack selected by options.Backend.
+//
+// gVisor (the default/"gvisor" backend) is a general purpose stack, built
+// with the semantics needed for a real, lossy public network in mind:
+// congestion control, SACK/RACK loss recovery, retransmission timers, etc.
+// TUN traffic instead travels over a local, kernel-to-userspace channel that
+// neither reorders nor drops packets in normal operation, so none of that
+// complexity is actually required to shuffle bytes between it and the
+// dispatcher. The "system" backend trades gVisor's generality for a much
+// smaller, more direct code path tailored to that trusted, in-order channel:
+// no congestion control, no SACK/RACK, minimal buffering, and a plain RTO
+// timer as a safety net for the rare real loss, rather than a full
+// re-implementation of one. See stack_system.go for details.
+func NewStack(ctx context.Context, options StackOptions, handler *Handler) (Stack, error) {
+	switch options.Backend {
+	case "", StackGVisor:
+		return newGVisorStack(ctx, options, handler)
+	case StackSystem:
+		return newSystemStack(ctx, options, handler)
+	default:
+		return nil, errors.New("unknown tun stack: ", options.Backend)
+	}
 }