After months of stonewalling against allegations by French prop trading firm Mosaic Finance, Eurex will shift from Ethernet frame limits to packet limits monitoring in its co-location network from April 2026. Markets participants tell us the change shuts down the ultra-low latency technique Mosaic Finance has labelled Corrupted Speculative Triggering (CST).
Ever since the outbreak of the controversy, Eurex has denied that CST exists but says Advanced Speculative Triggering is allowed. The two appellations stem from Mosaic work who changed the label of the practice from AST to CST in its dispute with Eurex.
Eurex has told clients via a circular authorised by Jonas Ullman, its COO, it will upgrade its co-location 2.0 monitoring infrastructure to Arista’s extensible operating system and a new MetaWatch release. At the same time, it modifies its general terms and conditions and replaces the current Ethernet frame limit with an Ethernet packet limit per order-entry line from 20 April 2026. The hard thresholds, 30,000 messages per second and 600,000 per minute per line, remain unchanged. They will now apply explicitly to all packets detected at the physical layer, including custom Physical Coding Sublayer (PCS) patterns. Sending traffic “in any manner” designed to bypass those limits is therefore explicitly prohibited.
In earlier complaints to the AMF, the Hessen exchange supervisor and ESMA, Mosaic’s president Hugues Morin described CST as a way for some high-frequency trading firms to pre-stream millions of malformed Ethernet packets over multiple order-entry connections. Mosaic alleges these packets, do not conform to Ethernet standards, and are allegedly invisible to the Arista 7130 MetaWatch counters that enforced Eurex’s frame cap, allowing CST users to exceed the 30,000-frame limit by several orders of magnitude while having to rent several multiple order entry lines to Eurex matching engine. In a document, published earlier this year Mosaic offered a way for Eurex to monitor and count these malformed packets with their current Arista setup.
Mosaic has said that when a genuine market-data packet finally arrived, CST / AST engines (FPGA or integrated circuits) could reuse the pre-sent preamble and start-of-frame delimiter as the first part of an order message, giving their orders a timestamp a few nanoseconds before the market-data event they were reacting to. At the speed of light, these racing orders start 1 to 2 meters in front of regular speculatively triggered orders. Mosaic’s lab tests seen by Global Trading suggested a gain of up to 6.4 nanoseconds and a fourfold increase in “hit ratio”, the probability of being first into the matching engine. Mosaic has said this advantage could have cost market participants up to €100 million of P&L.
Mosaic argued that CST / AST breached both the quantitative limits and the qualitative requirements of Eurex’s network access guide. It required participants to “use network protocols in an orderly fashion” and not to send corrupted Ethernet frames such as shortened or padded packets. Mosaïc supplied regulators with a MetaWatch configuration it said would allow Eurex to count corrupted packets and an expert opinion from the University of New Hampshire Inter operability Laboratory confirming that CST packets were not Ethernet-compliant.
Eurex, by contrast, had maintained in correspondence and court proceedings that the practice, which it described as Advanced Speculative Triggering (AST), was compliant as long as its existing monitoring did not trigger an alarm, and that AST packets did not reach the trading system but were discarded by switches. A case brought by Mosaic before the administrative court in Wiesbaden was rejected on procedural grounds, without a ruling on the substance of the allegations.
Read more: Eurex denies getting swamped by “corrupted” messages as HFT dispute escalates
Sources told us that whether this circular amounts to an effective ban of the practice will be easily verifiable in the nanosecond reaction data observable once the changes take place on 20 April 2026.
A Eurex Spokesman told Global trading: “The circular does not introduce a prohibition to Advanced Speculative Triggering (AST – not CST). It reflects our regular, continuous improvement of our monitoring infrastructure and clarifies how existing technology-related limits are enforced. Specifically, Eurex is extending the current Ethernet Frame Limit to an Ethernet Packet Limit, so that more network patterns are explicitly in scope of the same per second and per minute thresholds. ”
Mosaïc Finance declined to comment for this article.
