4 files changed, 48 insertions, 3 deletions
diff --git a/pkg/tcpip/header/ipv4.go b/pkg/tcpip/header/ipv4.go
index 7da4c4845..7b8034de4 100644
--- a/pkg/tcpip/header/ipv4.go
+++ b/pkg/tcpip/header/ipv4.go
@@ -85,6 +85,10 @@ const (
 	// units, the header cannot exceed 15*4 = 60 bytes.
 	IPv4MaximumHeaderSize = 60
 
+	// MinIPFragmentPayloadSize is the minimum number of payload bytes that
+	// the first fragment must carry when an IPv4 packet is fragmented.
+	MinIPFragmentPayloadSize = 8
+
 	// IPv4AddressSize is the size, in bytes, of an IPv4 address.
 	IPv4AddressSize = 4
 
diff --git a/pkg/tcpip/header/tcp.go b/pkg/tcpip/header/tcp.go
index 1141443bb..82cfe785c 100644
--- a/pkg/tcpip/header/tcp.go
+++ b/pkg/tcpip/header/tcp.go
@@ -176,6 +176,21 @@ const (
 
 	// TCPProtocolNumber is TCP's transport protocol number.
 	TCPProtocolNumber tcpip.TransportProtocolNumber = 6
+
+	// TCPMinimumMSS is the minimum acceptable value for MSS. This is the
+	// same as the value TCP_MIN_MSS defined net/tcp.h.
+	TCPMinimumMSS = IPv4MaximumHeaderSize + TCPHeaderMaximumSize + MinIPFragmentPayloadSize - IPv4MinimumSize - TCPMinimumSize
+
+	// TCPMaximumMSS is the maximum acceptable value for MSS.
+	TCPMaximumMSS = 0xffff
+
+	// TCPDefaultMSS is the MSS value that should be used if an MSS option
+	// is not received from the peer. It's also the value returned by
+	// TCP_MAXSEG option for a socket in an unconnected state.
+	//
+	// Per RFC 1122, page 85: "If an MSS option is not received at
+	// connection setup, TCP MUST assume a default send MSS of 536."
+	TCPDefaultMSS = 536
 )
 
 // SourcePort returns the "source port" field of the tcp header.
@@ -306,7 +321,7 @@ func ParseSynOptions(opts []byte, isAck bool) TCPSynOptions {
 	synOpts := TCPSynOptions{
 		// Per RFC 1122, page 85: "If an MSS option is not received at
 		// connection setup, TCP MUST assume a default send MSS of 536."
-		MSS: 536,
+		MSS: TCPDefaultMSS,
 		// If no window scale option is specified, WS in options is
 		// returned as -1; this is because the absence of the option
 		// indicates that the we cannot use window scaling on the
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index c61f96fb0..c4076666a 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -496,6 +496,10 @@ type AvailableCongestionControlOption string
 // buffer moderation.
 type ModerateReceiveBufferOption bool
 
+// MaxSegOption is used by SetSockOpt/GetSockOpt to set/get the current
+// Maximum Segment Size(MSS) value as specified using the TCP_MAXSEG option.
+type MaxSegOption int
+
 // MulticastTTLOption is used by SetSockOpt/GetSockOpt to control the default
 // TTL value for multicast messages. The default is 1.
 type MulticastTTLOption uint8
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index cb40fea94..beb90afb5 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -117,6 +117,7 @@ const (
 	notifyDrain
 	notifyReset
 	notifyKeepaliveChanged
+	notifyMSSChanged
 )
 
 // SACKInfo holds TCP SACK related information for a given endpoint.
@@ -218,8 +219,6 @@ type endpoint struct {
 	mu sync.RWMutex `state:"nosave"`
 	id stack.TransportEndpointID
 
-	// state             endpointState `state:".(endpointState)"`
-	// pState            ProtocolState
 	state EndpointState `state:".(EndpointState)"`
 
 	isPortReserved    bool `state:"manual"`
@@ -313,6 +312,10 @@ type endpoint struct {
 	// in SYN-RCVD state.
 	synRcvdCount int
 
+	// userMSS if non-zero is the MSS value explicitly set by the user
+	// for this endpoint using the TCP_MAXSEG setsockopt.
+	userMSS int
+
 	// The following fields are used to manage the send buffer. When
 	// segments are ready to be sent, they are added to sndQueue and the
 	// protocol goroutine is signaled via sndWaker.
@@ -917,6 +920,17 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		}
 		return nil
 
+	case tcpip.MaxSegOption:
+		userMSS := v
+		if userMSS < header.TCPMinimumMSS || userMSS > header.TCPMaximumMSS {
+			return tcpip.ErrInvalidOptionValue
+		}
+		e.mu.Lock()
+		e.userMSS = int(userMSS)
+		e.mu.Unlock()
+		e.notifyProtocolGoroutine(notifyMSSChanged)
+		return nil
+
 	case tcpip.ReceiveBufferSizeOption:
 		// Make sure the receive buffer size is within the min and max
 		// allowed.
@@ -1096,6 +1110,14 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 		e.lastErrorMu.Unlock()
 		return err
 
+	case *tcpip.MaxSegOption:
+		// This is just stubbed out. Linux never returns the user_mss
+		// value as it either returns the defaultMSS or returns the
+		// actual current MSS. Netstack just returns the defaultMSS
+		// always for now.
+		*o = header.TCPDefaultMSS
+		return nil
+
 	case *tcpip.SendBufferSizeOption:
 		e.sndBufMu.Lock()
 		*o = tcpip.SendBufferSizeOption(e.sndBufSize)