From 0f20090d78c318ca997af0e706729353ed611d74 Mon Sep 17 00:00:00 2001
From: soypat <graded.sp@gmail.com>
Date: Sat, 16 Aug 2025 18:26:50 -0300
Subject: [PATCH] bugfix: use ISS on active TCP connection

---
 examples/xnet/main.go | 3 ++-
 tcp/conn.go           | 4 ++++
 tcp/handler.go        | 5 +++--
 tcp/txqueue.go        | 4 +++-
 4 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/examples/xnet/main.go b/examples/xnet/main.go
index 66d8da9..f813765 100644
--- a/examples/xnet/main.go
+++ b/examples/xnet/main.go
@@ -203,8 +203,9 @@ func run() (err error) {
 		if err != nil {
 			return err
 		}
+		const tcpDebugTimeout = 60 * time.Minute
 		target := netip.AddrPortFrom(addrs[0], 80)
-		conn, err := rstack.DoDialTCP(uint16(softRand&0xefff)+1024, target, internetTimeout, internetRetries)
+		conn, err := rstack.DoDialTCP(uint16(softRand&0xefff)+1024, target, tcpDebugTimeout, internetRetries)
 		if err != nil {
 			return fmt.Errorf("TCP failed: %w", err)
 		}
diff --git a/tcp/conn.go b/tcp/conn.go
index 662cdff..539867b 100644
--- a/tcp/conn.go
+++ b/tcp/conn.go
@@ -17,6 +17,7 @@ import (
 var (
 	errDeadlineExceeded    = os.ErrDeadlineExceeded
 	errNoRemoteAddr        = errors.New("tcp: no remote address established")
+	errInvalidIP           = errors.New("tcp: invalid IP")
 	errMismatchedIPVersion = errors.New("mismatched IP version")
 )
 
@@ -70,6 +71,9 @@ func (conn *Conn) BufferedInput() int { return conn.h.BufferedInput() }
 // OpenActive opens a connection to a remote peer with a known IP address and port combination.
 // iss is the initial send sequence number which is ideally a random number which is far away from the last sequence number used on a connection to the same host.
 func (conn *Conn) OpenActive(localPort uint16, remote netip.AddrPort, iss Value) error {
+	if !remote.IsValid() {
+		return errInvalidIP
+	}
 	err := conn.h.OpenActive(localPort, remote.Port(), iss)
 	if err != nil {
 		return err
diff --git a/tcp/handler.go b/tcp/handler.go
index bcd339b..936e465 100644
--- a/tcp/handler.go
+++ b/tcp/handler.go
@@ -92,6 +92,7 @@ func (h *Handler) OpenActive(localPort, remotePort uint16, iss Value) error {
 		return errors.New("zero port on open call")
 	}
 	h.reset(localPort, remotePort, iss)
+	h.scb.SetRecvWindow(Size(h.bufRx.Size()))
 	return nil
 }
 
@@ -234,7 +235,7 @@ func (h *Handler) Send(b []byte) (int, error) {
 	var segment Segment
 	if h.AwaitingSynSend() {
 		// Handling init syn segment.
-		segment = ClientSynSegment(h.scb.ISS(), h.scb.RecvWindow())
+		segment = ClientSynSegment(h.bufTx.iss, Size(h.bufRx.Size()))
 		h.optcodec.PutOption16(b[sizeHeaderTCP:], OptMaxSegmentSize, uint16(len(b)))
 		offset++
 	} else {
@@ -266,7 +267,7 @@ func (h *Handler) Send(b []byte) (int, error) {
 	}
 	tfrm.SetSourcePort(h.localPort)
 	tfrm.SetDestinationPort(h.remotePort)
-	tfrm.SetSegment(segment, offset) // No TCP options.
+	tfrm.SetSegment(segment, offset)
 	tfrm.SetUrgentPtr(0)
 	return int(offset)*4 + int(segment.DATALEN), nil
 }
diff --git a/tcp/txqueue.go b/tcp/txqueue.go
index 9463773..479e579 100644
--- a/tcp/txqueue.go
+++ b/tcp/txqueue.go
@@ -32,6 +32,7 @@ type ringTx struct {
 	// seq     Value
 	// always empty ring.
 	emptyRing ringidx
+	iss       Value
 }
 
 // ringidx represents packet data inside RingTx
@@ -49,7 +50,7 @@ type ringidx struct {
 
 // Reset resets the RingTx's internal state to use buf as the main ring buffer and creates or reuses
 // the packet ring buffer.
-func (rtx *ringTx) Reset(buf []byte, maxqueuedPackets int, seq Value) error {
+func (rtx *ringTx) Reset(buf []byte, maxqueuedPackets int, iss Value) error {
 	buf = buf[:len(buf):len(buf)] // safely omit capacity section.
 	if maxqueuedPackets <= 0 {
 		return errors.New("queued packets <=0")
@@ -66,6 +67,7 @@ func (rtx *ringTx) Reset(buf []byte, maxqueuedPackets int, seq Value) error {
 	for i := range rtx.packets {
 		rtx.packets[i].markRcvd()
 	}
+	rtx.iss = iss
 	return nil
 }