3 files changed, 26 insertions, 6 deletions
diff --git a/pkg/tcpip/transport/tcp/accept.go b/pkg/tcpip/transport/tcp/accept.go
index 65c346046..1dd00d026 100644
--- a/pkg/tcpip/transport/tcp/accept.go
+++ b/pkg/tcpip/transport/tcp/accept.go
@@ -400,6 +400,9 @@ func (e *endpoint) acceptQueueIsFull() bool {
 // handleListenSegment is called when a listening endpoint receives a segment
 // and needs to handle it.
 func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
+	// TODO(b/143300739): Use the userMSS of the listening socket
+	// for accepted sockets.
+
 	switch s.flags {
 	case header.TCPFlagSyn:
 		opts := parseSynSegmentOptions(s)
@@ -434,13 +437,12 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 			//
 			// Enable Timestamp option if the original syn did have
 			// the timestamp option specified.
-			mss := mssForRoute(&s.route)
 			synOpts := header.TCPSynOptions{
 				WS:    -1,
 				TS:    opts.TS,
 				TSVal: tcpTimeStamp(timeStampOffset()),
 				TSEcr: opts.TSVal,
-				MSS:   uint16(mss),
+				MSS:   mssForRoute(&s.route),
 			}
 			e.sendSynTCP(&s.route, s.id, e.ttl, e.sendTOS, header.TCPFlagSyn|header.TCPFlagAck, cookie, s.sequenceNumber+1, ctx.rcvWnd, synOpts)
 			e.stack.Stats().TCP.ListenOverflowSynCookieSent.Increment()
diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 790e89cc3..ca982c451 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -442,7 +442,7 @@ func (h *handshake) execute() *tcpip.Error {
 
 	// Send the initial SYN segment and loop until the handshake is
 	// completed.
-	h.ep.amss = mssForRoute(&h.ep.route)
+	h.ep.amss = calculateAdvertisedMSS(h.ep.userMSS, h.ep.route)
 
 	synOpts := header.TCPSynOptions{
 		WS:            h.rcvWndScale,
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 6ca0d73a9..8234a8b53 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -411,7 +411,7 @@ type endpoint struct {
 
 	// userMSS if non-zero is the MSS value explicitly set by the user
 	// for this endpoint using the TCP_MAXSEG setsockopt.
-	userMSS int
+	userMSS uint16
 
 	// The following fields are used to manage the send buffer. When
 	// segments are ready to be sent, they are added to sndQueue and the
@@ -504,6 +504,21 @@ type endpoint struct {
 	stats Stats `state:"nosave"`
 }
 
+// calculateAdvertisedMSS calculates the MSS to advertise.
+//
+// If userMSS is non-zero and is not greater than the maximum possible MSS for
+// r, it will be used; otherwise, the maximum possible MSS will be used.
+func calculateAdvertisedMSS(userMSS uint16, r stack.Route) uint16 {
+	// The maximum possible MSS is dependent on the route.
+	maxMSS := mssForRoute(&r)
+
+	if userMSS != 0 && userMSS < maxMSS {
+		return userMSS
+	}
+
+	return maxMSS
+}
+
 // StopWork halts packet processing. Only to be used in tests.
 func (e *endpoint) StopWork() {
 	e.workMu.Lock()
@@ -752,7 +767,9 @@ func (e *endpoint) initialReceiveWindow() int {
 	if rcvWnd > math.MaxUint16 {
 		rcvWnd = math.MaxUint16
 	}
-	routeWnd := InitialCwnd * int(mssForRoute(&e.route)) * 2
+
+	// Use the user supplied MSS, if available.
+	routeWnd := InitialCwnd * int(calculateAdvertisedMSS(e.userMSS, e.route)) * 2
 	if rcvWnd > routeWnd {
 		rcvWnd = routeWnd
 	}
@@ -1206,7 +1223,7 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 			return tcpip.ErrInvalidOptionValue
 		}
 		e.mu.Lock()
-		e.userMSS = int(userMSS)
+		e.userMSS = uint16(userMSS)
 		e.mu.Unlock()
 		e.notifyProtocolGoroutine(notifyMSSChanged)
 		return nil
@@ -2383,5 +2400,6 @@ func (e *endpoint) Stats() tcpip.EndpointStats {
 }
 
 func mssForRoute(r *stack.Route) uint16 {
+	// TODO(b/143359391): Respect TCP Min and Max size.
 	return uint16(r.MTU() - header.TCPMinimumSize)
 }