net: enable multipath TCP by default for listeners

A previous change [1] was introduced to enable MPTCP by default
for both the clients and servers, based on the discussions [2] in
golang#56539, where MPTCP would be an opt-in for a release or
two, and then would become an opt-out.

This change was not accepted at the time because the support for
a few socket options was missing [3]. Now that this support has been
added [4] and backported to stable versions not to block MPTCP
deployment with Go, it sounds like a good time to reconsider the use
of MPTCP by default.

Instead of enabling MPTCP on both ends by default, as a first step,
it seems safer to change the default behaviour only for the server
side (Listeners). On the server side, the impact is minimal: when
clients don't request to use MPTCP, server applications will create
"plain" TCP sockets within the kernel when connections are accepted,
making the performance impact minimal. This should also ease
experiments where MPTCP is enabled by default on the client side
(Dialer).

The changes in this patch consist of a duplication of the mptcpStatus
enumeration to have both a mptcpStatusDial and a mptcpStatusListen,
where MPTCP is enabled by default in mptcpStatusListen, but disabled
by default in mptcpStatusDial. It is still possible to turn MPTCP support
on and off by using GODEBUG=multipathtcp=1.

[1] https://go-review.googlesource.com/c/go/+/563575
[2] https://go.dev/issue/56539#issuecomment-1309294637
[3] https://github.com/multipath-tcp/mptcp_net-next/issues/383
[4] bd11dc4fb9
[5] https://www.mptcp.dev/faq.html#why--when-should-mptcp-be-enabled-by-default

Updates #56539

Change-Id: I1ca0d6aaf74d3bda5468af135e29cdb405d3fd00
GitHub-Last-Rev: 5f9f29bfc1
GitHub-Pull-Request: golang/go#69016
Reviewed-on: https://go-review.googlesource.com/c/go/+/607715
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>

doc/godebug.md

@@ -157,6 +157,17 @@ no-op. This behavior is controlled by the `randseednop` setting.
 For Go 1.24 it defaults to `randseednop=1`.
 Using `randseednop=0` reverts to the pre-Go 1.24 behavior.
 
+Go 1.24 added new values for the `multipathtcp` setting.
+The possible values for `multipathtcp` are now:
+- "0": disable MPTCP on dialers and listeners by default
+- "1": enable MPTCP on dialers and listeners by default
+- "2": enable MPTCP on listeners only by default
+- "3": enable MPTCP on dialers only by default
+
+For Go 1.24, it now defaults to multipathtcp="2", thus
+enabled by default on listerners. Using multipathtcp="0" reverts to the
+pre-Go 1.24 behavior.
+
 ### Go 1.23
 
 Go 1.23 changed the channels created by package time to be unbuffered

doc/next/6-stdlib/99-minor/net/56539.md

@@ -0,0 +1,2 @@
+[ListenConfig] now uses MPTCP by default on systems where it is supported
+(currently on Linux only).

src/internal/godebugs/table.go

@@ -42,7 +42,7 @@ var All = []Info{
 	//{Name: "multipartfiles", Package: "mime/multipart"},
 	{Name: "multipartmaxheaders", Package: "mime/multipart"},
 	{Name: "multipartmaxparts", Package: "mime/multipart"},
-	{Name: "multipathtcp", Package: "net"},
+	{Name: "multipathtcp", Package: "net", Changed: 24, Old: "0"},
 	{Name: "netdns", Package: "net", Opaque: true},
 	{Name: "netedns0", Package: "net", Changed: 19, Old: "0"},
 	{Name: "panicnil", Package: "runtime", Changed: 21, Old: "1"},

src/net/dial.go

@@ -25,46 +25,93 @@ const (
 	// defaultTCPKeepAliveCount is a default constant value for TCP_KEEPCNT.
 	defaultTCPKeepAliveCount = 9
 
-	// For the moment, MultiPath TCP is not used by default
+	// For the moment, MultiPath TCP is used by default with listeners, if
+	// available, but not with dialers.
 	// See go.dev/issue/56539
-	defaultMPTCPEnabled = false
+	defaultMPTCPEnabledListen = true
+	defaultMPTCPEnabledDial   = false
 )
 
+// The type of service offered
+//
+//	0 == MPTCP disabled
+//	1 == MPTCP enabled
+//	2 == MPTCP enabled on listeners only
+//	3 == MPTCP enabled on dialers only
 var multipathtcp = godebug.New("multipathtcp")
 
-// mptcpStatus is a tristate for Multipath TCP, see go.dev/issue/56539
-type mptcpStatus uint8
+// mptcpStatusDial is a tristate for Multipath TCP on clients,
+// see go.dev/issue/56539
+type mptcpStatusDial uint8
 
 const (
-	// The value 0 is the system default, linked to defaultMPTCPEnabled
-	mptcpUseDefault mptcpStatus = iota
-	mptcpEnabled
-	mptcpDisabled
+	// The value 0 is the system default, linked to defaultMPTCPEnabledDial
+	mptcpUseDefaultDial mptcpStatusDial = iota
+	mptcpEnabledDial
+	mptcpDisabledDial
 )
 
-func (m *mptcpStatus) get() bool {
+func (m *mptcpStatusDial) get() bool {
 	switch *m {
-	case mptcpEnabled:
+	case mptcpEnabledDial:
 		return true
-	case mptcpDisabled:
+	case mptcpDisabledDial:
 		return false
 	}
 
 	// If MPTCP is forced via GODEBUG=multipathtcp=1
-	if multipathtcp.Value() == "1" {
+	if multipathtcp.Value() == "1" || multipathtcp.Value() == "3" {
 		multipathtcp.IncNonDefault()
 
 		return true
 	}
 
-	return defaultMPTCPEnabled
+	return defaultMPTCPEnabledDial
 }
 
-func (m *mptcpStatus) set(use bool) {
+func (m *mptcpStatusDial) set(use bool) {
 	if use {
-		*m = mptcpEnabled
+		*m = mptcpEnabledDial
 	} else {
-		*m = mptcpDisabled
+		*m = mptcpDisabledDial
+	}
+}
+
+// mptcpStatusListen is a tristate for Multipath TCP on servers,
+// see go.dev/issue/56539
+type mptcpStatusListen uint8
+
+const (
+	// The value 0 is the system default, linked to defaultMPTCPEnabledListen
+	mptcpUseDefaultListen mptcpStatusListen = iota
+	mptcpEnabledListen
+	mptcpDisabledListen
+)
+
+func (m *mptcpStatusListen) get() bool {
+	switch *m {
+	case mptcpEnabledListen:
+		return true
+	case mptcpDisabledListen:
+		return false
+	}
+
+	// If MPTCP is disabled via GODEBUG=multipathtcp=0 or only
+	// enabled on dialers, but not on listeners.
+	if multipathtcp.Value() == "0" || multipathtcp.Value() == "3" {
+		multipathtcp.IncNonDefault()
+
+		return false
+	}
+
+	return defaultMPTCPEnabledListen
+}
+
+func (m *mptcpStatusListen) set(use bool) {
+	if use {
+		*m = mptcpEnabledListen
+	} else {
+		*m = mptcpDisabledListen
 	}
 }
 
@@ -175,7 +222,7 @@ type Dialer struct {
 	// If mptcpStatus is set to a value allowing Multipath TCP (MPTCP) to be
 	// used, any call to Dial with "tcp(4|6)" as network will use MPTCP if
 	// supported by the operating system.
-	mptcpStatus mptcpStatus
+	mptcpStatus mptcpStatusDial
 }
 
 func (d *Dialer) dualStack() bool { return d.FallbackDelay >= 0 }
@@ -720,7 +767,7 @@ type ListenConfig struct {
 	// If mptcpStatus is set to a value allowing Multipath TCP (MPTCP) to be
 	// used, any call to Listen with "tcp(4|6)" as network will use MPTCP if
 	// supported by the operating system.
-	mptcpStatus mptcpStatus
+	mptcpStatus mptcpStatusListen
 }
 
 // MultipathTCP reports whether MPTCP will be used.