Congestion avoidance - Revision history

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Dimitri at 20:43, 19 July 2026

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	{{DISPLAYTITLE:mb-ecn - Convergence Analysis}}		{{DISPLAYTITLE:mb-ecn - Convergence Analysis}}

	This ~~is the companion to [[mb-ecn]] (doc/mb-ecn.mediawiki). That~~		This document says what can be '''proven''' about mb-ecn converging, and at what level of idealization.
	document ~~says what the algorithm does; this one~~ says what can be
	'''proven''' about it converging, and at what level of idealization.
	It is written for engineers and non-expert mathematicians: every proof		It is written for engineers and non-expert mathematicians: every proof
	technique gets a plain-language explanation of what it buys, and every		technique gets a plain-language explanation of what it buys, and every

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	* <code>src/ipcpd/unicast/ca/mb-ecn.c</code> — rate law (<code>mb_ecn_increase</code>, <code>mb_ecn_decrease</code>, <code>mb_ecn_washout</code>), receiver estimator, pacer.		* <code>src/ipcpd/unicast/ca/mb-ecn.c</code> — rate law (<code>mb_ecn_increase</code>, <code>mb_ecn_decrease</code>, <code>mb_ecn_washout</code>), receiver estimator, pacer.
	* <code>src/ipcpd/unicast/ca/tests/mb_ecn_test.c</code> — pins the constants and invariants used above (incl. <code>test_mb_ecn_decrease_subms_carry</code> for the exact-time property).		* <code>src/ipcpd/unicast/ca/tests/mb_ecn_test.c</code> — pins the constants and invariants used above (incl. <code>test_mb_ecn_decrease_subms_carry</code> for the exact-time property).
	* doc/mb-ecn.mediawiki — the algorithm document; notation and parameter table shared with this one.

	=== Literature ===		=== Literature ===

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Dimitri at 22:23, 11 July 2026

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	RFC 8174) says, but only where they appear in capitals.		RFC 8174) says, but only where they appear in capitals.

	Note: Co-authered by Claude Fable 5.		Note: Co-authered by Anthropic Claude Fable 5.

	== Notation ==		== Notation ==

Dimitri at 22:22, 11 July 2026

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	RFC 8174) says, but only where they appear in capitals.		RFC 8174) says, but only where they appear in capitals.

			Note: Co-authered by Claude Fable 5.

	== Notation ==		== Notation ==

Dimitri at 22:14, 11 July 2026

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	in the IPCP, once per layer. Within a layer the IPCP controls		in the IPCP, once per layer. Within a layer the IPCP controls
	'''aggregates''': one control loop per (destination address, QoS cube),		'''aggregates''': one control loop per (destination address, QoS cube),
	shared by every flow to that destination~~. The other policy is~~		shared by every flow to that destination.
	~~<code>nop</code>, which does nothing~~.

	This document is about the algorithm: what it does and why. The		This document is about the algorithm: what it does and why. The

Dimitri at 22:13, 11 July 2026

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Dimitri at 21:25, 11 July 2026

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	== Notation ==		== Notation ==

	;<code>r</code>		{\| class="wikitable"
	:Paced send rate~~, bytes per second.~~		! Symbol !! Units !! Meaning
	;<code>C</code>		\|-
	:Bottleneck capacity~~, bytes per second.~~		\| <code>r</code> \|\| B/s \|\| Paced send rate
	;<code>S</code>		\|-
	:Packet size ~~in bytes.~~		\| <code>C</code> \|\| B/s \|\| Bottleneck capacity
	;<code>q</code>		\|-
	:Standing queue occupancy at the bottleneck~~, in packets.~~		\| <code>S</code> \|\| bytes \|\| Packet size
	;<code>Q</code>		\|-
	:Marking quantum, packets per mark unit (4).		\| <code>q</code> \|\| packets \|\| Standing queue occupancy at the bottleneck
	;<code>ecn</code>		\|-
	:Per-packet congestion mark set by a forwarder~~, an unsigned integer~~		\| <code>Q</code> \|\| packets \|\| Marking quantum: packets per mark unit (4)
	~~(on-wire field is 8~~-~~bit).~~		\|-
	;<code>ece</code>		\| <code>ecn</code> \|\| 8-bit code \|\| Per-packet congestion mark set by a forwarder
	:Receiver-side congestion estimate~~, fixed point with LSB = 1/32~~ of an		\|-
	<code>ecn</code> ~~unit, i.e. <code>ece = 32 × (time~~-~~mean ecn)</code>.~~		\| <code>ece</code> \|\| 32 × ecn \|\| Receiver-side congestion estimate (time-mean of <code>ecn</code>)
	;<code>cap8</code>		\|-
	:Path-capacity ~~code carried per packet~~, quarter-log2:		\| <code>cap8</code> \|\| 8-bit code \|\| Path capacity, quarter-log2: <code>C ≈ 2^(cap8 / 4)</code> B/s, 0 = unknown
	<code>C ≈ 2^(cap8 / 4)</code> ~~bytes~~/s, 0 = unknown.		\|-
	;<code>W</code>		\| <code>W</code> \|\| ns \|\| Receiver averaging window
	:Receiver averaging window~~, nanoseconds.~~		\|-
	;<code>a</code>		\| <code>a</code> \|\| B/s^2 \|\| Additive-increase slope
	:Additive-increase slope~~, bytes/s per second.~~		\|-
	;<code>M</code>		\| <code>M</code> \|\| <code>ece</code> \|\| Full-congestion reference level (512)
	~~:Full-congestion reference level of~~ <code>ece</code> (512).		\|-
	;<code>Δt</code>		\| <code>Δt</code> \|\| ns \|\| Elapsed wall-clock time between two events
	:Elapsed wall-clock time between two events~~, nanoseconds.~~		\|-
	;<code>τ</code>		\| <code>τ</code> \|\| — \|\| Path round-trip time (feedback delay); the controller does not measure it
	:Path round-trip time (feedback delay)~~. The~~ controller does not		\|}
	measure it.

	Rate is the control variable throughout.		Rate is the control variable throughout.