Handle TCP segment split cases as per MSS.

- Always split segments larger than MSS.
  Currently, we base the segment split decision as a function of the
  send congestion window and MSS, which could be greater than the MSS
  advertised by remote.
- While splitting segments, ensure the PSH flag is reset when there
  are segments that are queued to be sent.
- With TCP_CORK, hold up segments up until MSS. Fix a bug in computing
  available send space before attempting to coalesce segments.

Fixes #2832

PiperOrigin-RevId: 314802928

2 files changed, 54 insertions, 3 deletions

diff --git a/test/packetimpact/testbench/connections.go b/test/packetimpact/testbench/connections.go
index fb32964e9..6e85d6fab 100644
--- a/test/packetimpact/testbench/connections.go
+++ b/test/packetimpact/testbench/connections.go
@@ -628,15 +628,36 @@ func NewTCPIPv4(t *testing.T, outgoingTCP, incomingTCP TCP) TCPIPv4 {
 	}
 }
 
-// Handshake performs a TCP 3-way handshake. The input Connection should have a
+// Connect performs a TCP 3-way handshake. The input Connection should have a
 // final TCP Layer.
-func (conn *TCPIPv4) Handshake() {
+func (conn *TCPIPv4) Connect() {
+	conn.t.Helper()
+
 	// Send the SYN.
 	conn.Send(TCP{Flags: Uint8(header.TCPFlagSyn)})
 
 	// Wait for the SYN-ACK.
 	synAck, err := conn.Expect(TCP{Flags: Uint8(header.TCPFlagSyn | header.TCPFlagAck)}, time.Second)
-	if synAck == nil {
+	if err != nil {
+		conn.t.Fatalf("didn't get synack during handshake: %s", err)
+	}
+	conn.layerStates[len(conn.layerStates)-1].(*tcpState).synAck = synAck
+
+	// Send an ACK.
+	conn.Send(TCP{Flags: Uint8(header.TCPFlagAck)})
+}
+
+// ConnectWithOptions performs a TCP 3-way handshake with given TCP options.
+// The input Connection should have a final TCP Layer.
+func (conn *TCPIPv4) ConnectWithOptions(options []byte) {
+	conn.t.Helper()
+
+	// Send the SYN.
+	conn.Send(TCP{Flags: Uint8(header.TCPFlagSyn), Options: options})
+
+	// Wait for the SYN-ACK.
+	synAck, err := conn.Expect(TCP{Flags: Uint8(header.TCPFlagSyn | header.TCPFlagAck)}, time.Second)
+	if err != nil {
 		conn.t.Fatalf("didn't get synack during handshake: %s", err)
 	}
 	conn.layerStates[len(conn.layerStates)-1].(*tcpState).synAck = synAck
@@ -656,6 +677,31 @@ func (conn *TCPIPv4) ExpectData(tcp *TCP, payload *Payload, timeout time.Duratio
 	return (*Connection)(conn).ExpectFrame(expected, timeout)
 }
 
+// ExpectNextData attempts to receive the next incoming segment for the
+// connection and expects that to match the given layers.
+//
+// It differs from ExpectData() in that here we are only interested in the next
+// received segment, while ExpectData() can receive multiple segments for the
+// connection until there is a match with given layers or a timeout.
+func (conn *TCPIPv4) ExpectNextData(tcp *TCP, payload *Payload, timeout time.Duration) (Layers, error) {
+	// Receive the first incoming TCP segment for this connection.
+	got, err := conn.ExpectData(&TCP{}, nil, timeout)
+	if err != nil {
+		return nil, err
+	}
+
+	expected := make([]Layer, len(conn.layerStates))
+	expected[len(expected)-1] = tcp
+	if payload != nil {
+		expected = append(expected, payload)
+		tcp.SeqNum = Uint32(uint32(*conn.RemoteSeqNum()) - uint32(payload.Length()))
+	}
+	if !(*Connection)(conn).match(expected, got) {
+		return nil, fmt.Errorf("next frame is not matching %s during %s: got %s", expected, timeout, got)
+	}
+	return got, nil
+}
+
 // Send a packet with reasonable defaults. Potentially override the TCP layer in
 // the connection with the provided layer and add additionLayers.
 func (conn *TCPIPv4) Send(tcp TCP, additionalLayers ...Layer) {
diff --git a/test/packetimpact/testbench/layers.go b/test/packetimpact/testbench/layers.go
index 1b0e5b8fc..560c4111b 100644
--- a/test/packetimpact/testbench/layers.go
+++ b/test/packetimpact/testbench/layers.go
@@ -939,6 +939,11 @@ func (l *Payload) ToBytes() ([]byte, error) {
 	return l.Bytes, nil
 }
 
+// Length returns payload byte length.
+func (l *Payload) Length() int {
+	return l.length()
+}
+
 func (l *Payload) match(other Layer) bool {
 	return equalLayer(l, other)
 }