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Pat Whittingslow
3eb1f39f1d
* feat(tcp): implement window scaling (RFC 7323) The window-scale option was parsed for length validation but never applied: the effective receive window was capped at the 16-bit field, which caps throughput at 65535/RTT (about 4MB/s on a 15ms path) no matter how large the receive buffer is. Worse, a receive buffer over 64KiB silently WRAPPED the advertised window on the SYN (uint16 truncation): a 128KiB buffer went on the wire as a near-zero window. Design: scaling lives purely at the wire seam in Handler. The ControlBlock always holds real octet counts; conversion happens on frame read (peer windows shifted up by the peer's offer, never on SYN segments) and frame write (wireWnd: our shift down, SYN never scaled, saturation instead of wrap when the value still does not fit). The local shift derives from the receive buffer size in SetBuffers; every active SYN offers it (a zero shift still lets the peer scale, RFC 7323 §2.5) and a SYN-ACK echoes it only when the peer's SYN carried the option. The ControlBlock's three 2**16 window caps move to the scaled maximum (65535<<14). Tests: on-wire negotiation with asymmetric buffers (shift values, unscaled saturated SYN windows, first scaled advertisement, peer scaling back up); a transfer proving more than 64KiB genuinely in flight without a single ACK, received intact; wire-safety corners (no echo without an offer, saturation not wrap, so the pre-existing 128KiB SYN wrap bug stays pinned). Fuzzers clean: 9.1M TCB execs, 4.9M full-stack HTTP execs, 7.2M TCB actions. * use min instead of if statement * omitting use of min saves 8 bytes * fix option offset bug and add OptionCodec.Next method --------- Co-authored-by: Derek den Haas <d.haas@directcode.com>
34 lines
1.9 KiB
Go
34 lines
1.9 KiB
Go
package tcp
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// TransmitUnlimited size returned by [Policy.PreTx] to signal no new data transmit limit (no congestion control).
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const TransmitUnlimited = ^Size(0)
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// Policy observes segment traffic and steers transmit behaviour: RTO,
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// congestion control and the like (discussion #157). The tcp package holds no
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// clock, so a Policy needing time carries its own (issue #140).
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type Policy interface {
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// Reset returns the Policy to its pre-connection state. Should be called on every
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// Open/Listen on connection creation. Configuration like clock setting and fine tuning
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// should persist throughout the Policy lifetime after Reset calls.
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Reset()
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// PreTx is called before writing to a frame.
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// The outgoing frame options can be set by the Policy and will be respected if Frame offset >5.
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// Keep in mind Handler will add options PreTx already added, these options are best overwritten in PostTx.
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// retransmitFrom is ignored unless within [snd.UNA, snd.NXT] and returned retransmit==true.
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// newTransmitLimit sets the maximum number of new bytes to send over the wire (congestion control).
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// If not implementing congestion control then newTransmitLimit=[TransmitUnlimited].
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PreTx(h *Handler, outgoingOpts Frame) (newTransmitLimit Size, retransmitFrom Value, retransmit bool)
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// PostTx called on leaving the transmit path with the fully written frame.
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// PostTx can strategically overwrite options normally set by Handler like MSS, Window scaling which
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// ends up being more ergonomic than adding them in PreTx and then de-duplicating them in PostTx.
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PostTx(h *Handler, outgoing Frame)
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// PreRx is called by [Handler] on every incoming segment.
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// PreRx can choose to drop segment if it returns keep=false.
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PreRx(h *Handler, incoming Frame) (keep bool)
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// PostRx is called by [Handler] after accepting an incoming segment.
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// To access [ControlBlock.SendUNA] before incoming frame was processed save UNA in PreRx.
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PostRx(h *Handler, prevState State, accepted Frame)
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}
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