Stub support for TCP_SYNCNT and TCP_WINDOW_CLAMP.

This change adds support for TCP_SYNCNT and TCP_WINDOW_CLAMP options
in GetSockOpt/SetSockOpt. This change does not really change any
behaviour in Netstack and only stores/returns the stored value.

Actual honoring of these options will be added as required.

Fixes #2626, #2625

PiperOrigin-RevId: 311453777

2 files changed, 83 insertions, 2 deletions

diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 07d3e64c8..71735029e 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -470,6 +470,17 @@ type endpoint struct {
 	// for this endpoint using the TCP_MAXSEG setsockopt.
 	userMSS uint16
 
+	// maxSynRetries is the maximum number of SYN retransmits that TCP should
+	// send before aborting the attempt to connect. It cannot exceed 255.
+	//
+	// NOTE: This is currently a no-op and does not change the SYN
+	// retransmissions.
+	maxSynRetries uint8
+
+	// windowClamp is used to bound the size of the advertised window to
+	// this value.
+	windowClamp uint32
+
 	// 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.
@@ -795,8 +806,10 @@ func newEndpoint(s *stack.Stack, netProto tcpip.NetworkProtocolNumber, waiterQue
 			interval: 75 * time.Second,
 			count:    9,
 		},
-		uniqueID: s.UniqueID(),
-		txHash:   s.Rand().Uint32(),
+		uniqueID:      s.UniqueID(),
+		txHash:        s.Rand().Uint32(),
+		windowClamp:   DefaultReceiveBufferSize,
+		maxSynRetries: DefaultSynRetries,
 	}
 
 	var ss SendBufferSizeOption
@@ -829,6 +842,11 @@ func newEndpoint(s *stack.Stack, netProto tcpip.NetworkProtocolNumber, waiterQue
 		e.tcpLingerTimeout = time.Duration(tcpLT)
 	}
 
+	var synRetries tcpip.TCPSynRetriesOption
+	if err := s.TransportProtocolOption(ProtocolNumber, &synRetries); err == nil {
+		e.maxSynRetries = uint8(synRetries)
+	}
+
 	if p := s.GetTCPProbe(); p != nil {
 		e.probe = p
 	}
@@ -1603,6 +1621,36 @@ func (e *endpoint) SetSockOptInt(opt tcpip.SockOptInt, v int) *tcpip.Error {
 		e.ttl = uint8(v)
 		e.UnlockUser()
 
+	case tcpip.TCPSynCountOption:
+		if v < 1 || v > 255 {
+			return tcpip.ErrInvalidOptionValue
+		}
+		e.LockUser()
+		e.maxSynRetries = uint8(v)
+		e.UnlockUser()
+
+	case tcpip.TCPWindowClampOption:
+		if v == 0 {
+			e.LockUser()
+			switch e.EndpointState() {
+			case StateClose, StateInitial:
+				e.windowClamp = 0
+				e.UnlockUser()
+				return nil
+			default:
+				e.UnlockUser()
+				return tcpip.ErrInvalidOptionValue
+			}
+		}
+		var rs ReceiveBufferSizeOption
+		if err := e.stack.TransportProtocolOption(ProtocolNumber, &rs); err == nil {
+			if v < rs.Min/2 {
+				v = rs.Min / 2
+			}
+		}
+		e.LockUser()
+		e.windowClamp = uint32(v)
+		e.UnlockUser()
 	}
 	return nil
 }
@@ -1826,6 +1874,18 @@ func (e *endpoint) GetSockOptInt(opt tcpip.SockOptInt) (int, *tcpip.Error) {
 		e.UnlockUser()
 		return v, nil
 
+	case tcpip.TCPSynCountOption:
+		e.LockUser()
+		v := int(e.maxSynRetries)
+		e.UnlockUser()
+		return v, nil
+
+	case tcpip.TCPWindowClampOption:
+		e.LockUser()
+		v := int(e.windowClamp)
+		e.UnlockUser()
+		return v, nil
+
 	default:
 		return -1, tcpip.ErrUnknownProtocolOption
 	}
diff --git a/pkg/tcpip/transport/tcp/protocol.go b/pkg/tcpip/transport/tcp/protocol.go
index cfd9a4e8e..1e64283b5 100644
--- a/pkg/tcpip/transport/tcp/protocol.go
+++ b/pkg/tcpip/transport/tcp/protocol.go
@@ -64,6 +64,10 @@ const (
 	// DefaultTCPTimeWaitTimeout is the amount of time that sockets linger
 	// in TIME_WAIT state before being marked closed.
 	DefaultTCPTimeWaitTimeout = 60 * time.Second
+
+	// DefaultSynRetries is the default value for the number of SYN retransmits
+	// before a connect is aborted.
+	DefaultSynRetries = 6
 )
 
 // SACKEnabled option can be used to enable SACK support in the TCP
@@ -164,6 +168,7 @@ type protocol struct {
 	tcpTimeWaitTimeout         time.Duration
 	minRTO                     time.Duration
 	synRcvdCount               synRcvdCounter
+	synRetries                 uint8
 	dispatcher                 *dispatcher
 }
 
@@ -346,6 +351,15 @@ func (p *protocol) SetOption(option interface{}) *tcpip.Error {
 		p.mu.Unlock()
 		return nil
 
+	case tcpip.TCPSynRetriesOption:
+		if v < 1 || v > 255 {
+			return tcpip.ErrInvalidOptionValue
+		}
+		p.mu.Lock()
+		p.synRetries = uint8(v)
+		p.mu.Unlock()
+		return nil
+
 	default:
 		return tcpip.ErrUnknownProtocolOption
 	}
@@ -420,6 +434,12 @@ func (p *protocol) Option(option interface{}) *tcpip.Error {
 		p.mu.RUnlock()
 		return nil
 
+	case *tcpip.TCPSynRetriesOption:
+		p.mu.RLock()
+		*v = tcpip.TCPSynRetriesOption(p.synRetries)
+		p.mu.RUnlock()
+		return nil
+
 	default:
 		return tcpip.ErrUnknownProtocolOption
 	}
@@ -452,6 +472,7 @@ func NewProtocol() stack.TransportProtocol {
 		tcpTimeWaitTimeout:         DefaultTCPTimeWaitTimeout,
 		synRcvdCount:               synRcvdCounter{threshold: SynRcvdCountThreshold},
 		dispatcher:                 newDispatcher(runtime.GOMAXPROCS(0)),
+		synRetries:                 DefaultSynRetries,
 		minRTO:                     MinRTO,
 	}
 }