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Tcp rst handling (#40)

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* claude suggests a way forward

* add timing to capture printer

* add pcap.Flags

* fix ICMP CRC calculation and add test

* bugfix: still send data on half-close state(close-wait)

* fix pcap test
This commit is contained in:
Pat Whittingslow
2026-02-23 13:50:13 +01:00
committed by GitHub
parent 08423d0dba
commit 7d323aae19
10 changed files with 386 additions and 46 deletions
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tcp/control.go
+16 -14
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@@ -183,8 +183,9 @@ func (tcb *ControlBlock) PendingSegment(payloadLen int) (_ Segment, ok bool) {
}
pending := tcb.pending[0]
established := tcb._state == StateEstablished
if !established && tcb._state != StateCloseWait {
payloadLen = 0 // Can't send data if not established.
canSendData := established || tcb._state == StateCloseWait
if !canSendData {
payloadLen = 0 // Can't send data if not established or close-wait.
}
if pending == 0 && payloadLen == 0 {
return Segment{}, false // No pending segment.
@@ -206,7 +207,7 @@ func (tcb *ControlBlock) PendingSegment(payloadLen int) (_ Segment, ok bool) {
pending |= FlagPSH // By default ensure all data flushed to destination application immediately on receive.
}
if established {
if canSendData {
pending |= FlagACK // ACK is always set in established state. Not in RFC9293 but somehow expected?
} else {
payloadLen = 0 // Can't send data if not established.
@@ -420,7 +421,7 @@ func (tcb *ControlBlock) validateIncomingSegment(seg Segment) (err error) {
case checkSEQ && !seg.Last().InWindow(tcb.rcv.NXT, tcb.rcv.WND) && !zeroWindowOK:
err = errLastNotInWindow
case checkSEQ && seg.SEQ != tcb.rcv.NXT:
case checkSEQ && !flags.HasAny(FlagRST) && seg.SEQ != tcb.rcv.NXT:
// This part diverts from TCB as described in RFC 9293. We want to support
// only sequential segments to keep implementation simple and maintainable. See SHLD-31.
err = errRequireSequential
@@ -485,22 +486,23 @@ func (tcb *ControlBlock) resetRcv(localWND Size, remoteISS Value) {
func (tcb *ControlBlock) handleRST(seq Value) error {
tcb.debug("rcv:RST", slog.String("state", tcb._state.String()))
if seq != tcb.rcv.NXT {
// See RFC9293: If the RST bit is set and the sequence number does not exactly match the next expected sequence value, yet is within the current receive window, TCP endpoints MUST send an acknowledgment (challenge ACK).
tcb.challengeAck = true
tcb.pending[0] |= FlagACK
return errDropSegment
}
if tcb._state.IsPreestablished() {
// RFC 9293 §3.5.3: non-synchronized states accept RST if SEQ is in window.
// No challenge ACK for non-synchronized states. Return to LISTEN.
tcb.pending[0] = 0
tcb._state = StateListen
tcb.resetSnd(tcb.snd.ISS+tcb.rstJump(), tcb.snd.WND)
tcb.resetRcv(tcb.rcv.WND, 3_14159_2653^tcb.rcv.IRS)
} else {
tcb.Abort() // Enter closed state and return.
return net.ErrClosed
return errDropSegment
}
return errDropSegment
// Synchronized states: exact match required, challenge ACK for in-window non-exact.
if seq != tcb.rcv.NXT {
tcb.challengeAck = true
tcb.pending[0] |= FlagACK
return errDropSegment
}
tcb.Abort()
return net.ErrClosed
}
func (tcb *ControlBlock) rstJump() Value {