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tcp: simplified Policy implementation based on @MDr164 (#190)

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* begin prepping policy refactor manually

* implement tcp.Policy and refactor rto to use it

* fix CI

* chatting with claude gave me idea to reformulate Policy

* tcp.Policy: add newTransmitLimit output

* merge with main and fix failing tests

* remove fix.patch

* answer my own comments
This commit is contained in:
Pat Whittingslow
2026-09-07 10:46:24 -03:00
committed by GitHub
parent d7f3924489
commit 07afcfd924
14 changed files with 1518 additions and 806 deletions
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tcp/control.go
+29 -3
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@@ -95,6 +95,12 @@ func (tcb *ControlBlock) RecvWindow() Size { return tcb.rcv.WND }
// ISS returns the initial sequence number of the connection that was defined on a call to Open by user.
func (tcb *ControlBlock) ISS() Value { return tcb.snd.ISS }
// SendUNA returns snd.UNA, the oldest sequence number not yet acked by the remote.
func (tcb *ControlBlock) SendUNA() Value { return tcb.snd.UNA }
// SendNext returns snd.NXT, one past the highest sequence number sent.
func (tcb *ControlBlock) SendNext() Value { return tcb.snd.NXT }
// MaxInFlightData returns the maximum size of a segment that can be sent by taking into account
// the send window size and the unacked data. Returns 0 before StateSynRcvd.
func (tcb *ControlBlock) MaxInFlightData() Size {
@@ -257,14 +263,34 @@ func (tcb *ControlBlock) HasPendingRetransmit() bool {
return tcb._state.TxDataOpen() && tcb.dupack >= retransmitAfterDupacks && tcb.nRetransmit <= tcb.dupack-retransmitAfterDupacks
}
// RetransmitFrom rewinds snd.NXT back to newNxt so the next PendingSegment and
// Send calls retransmit unacknowledged data from that sequence number onwards.
// It must be paired with ringTx.RetransmitFrom to rewind the transmit buffer to
// the same point. Implements RFC 9293 §3.10.8 (RETRANSMISSION TIMEOUT).
//
// It reports false and changes nothing when newNxt falls outside the
// unacknowledged range [snd.UNA, snd.NXT] or the connection cannot send data, so
// a misbehaving [Policy] cannot corrupt the send sequence space.
func (tcb *ControlBlock) RetransmitFrom(newNxt Value) bool {
if !tcb._state.txQueuedDataOpen() {
// Matches [State.TxDataOpen] and other states that may have data queued to make progress.
// Matches [ControlBlock.PendingSegment] gate (RFC 9293 §3.10.8).
return false
} else if newNxt.LessThan(tcb.snd.UNA) || tcb.snd.NXT.LessThan(newNxt) {
return false
}
tcb.snd.NXT = newNxt
tcb.dupack = 0
tcb.nRetransmit = 0
return true
}
// RetransmitAll rewinds snd.NXT back to snd.UNA so the next PendingSegment and
// Send calls retransmit all unacknowledged data from the oldest sequence number
// (go-back-N). It must be paired with ringTx.RetransmitFromUNA to rewind the
// transmit buffer. Implements RFC 9293 §3.10.8 (RETRANSMISSION TIMEOUT).
func (tcb *ControlBlock) RetransmitAll() {
tcb.snd.NXT = tcb.snd.UNA
tcb.dupack = 0
tcb.nRetransmit = 0
tcb.RetransmitFrom(tcb.snd.UNA)
}
// PendingSegment calculates a suitable next segment to send from a payload length.