Ultra Ethernet — UEC 1.0 for AI Fabrics
Ultra Ethernet is the industry's open answer to the question: can Ethernet replace InfiniBand for trillion-parameter training? The Ultra Ethernet Consortium 1.0 specification redesigns the transport — packet spray, multi-path RDMA, out-of-order delivery, modern congestion control. OcNOS provides the standards-based fabric layer that UEC NICs require.
Packet-Spray Across Every Path
A single RDMA message between two GPUs is sprayed packet-by-packet across all four spine paths simultaneously. The fabric tolerates out-of-order arrival; the destination UEC NIC reassembles in order. No flow-pinning, no hash collisions, no idle uplinks.
Why UEC matters now
RoCEv2 was the first generation of "RDMA over Ethernet". It works — but it inherits the design constraints of TCP/IP over Ethernet from a different era. Single flow per (5-tuple, hash) means GPU collectives can saturate one path while parallel paths sit idle. PFC pause propagation can cause head-of-line blocking and, in pathological topologies, deadlock. NACK-based loss recovery puts a tax on tail latency.
The Ultra Ethernet Consortium — backed by AMD, Arista, Broadcom, Cisco, HPE, Intel, Meta, Microsoft, Oracle, and others — released the UEC 1.0 specification to redesign the transport for AI scale. It keeps the standard physical Ethernet you can buy from anyone, and replaces the upper layers with a transport built for 1k–100k GPU jobs.
The four UEC pillars
Packet spray
A single message is sprayed across every available path. No flow pinning. No ECMP collisions. Fabric utilisation approaches the theoretical maximum.
Out-of-order delivery
The fabric is allowed to reorder. The UEC NIC reassembles in order before delivery, so applications and the GPU never see disorder.
Modern congestion control
Sender-based and receiver-driven schemes replace coarse PFC pause as the primary tool. Pause becomes a backstop, not the front line.
Selective retransmit
Lost packets are recovered selectively rather than restarting the RDMA window — collapsing the tail-latency penalty of any in-fabric loss.
The OcNOS UEC implementation
UEC 1.0 fabric profile
OcNOS-DC implements the UEC 1.0 fabric-side profile — the configuration and behaviours that switches must support to interoperate with UEC-compliant NICs.
OOO-tolerant forwarding
Per-packet ECMP, packet-spray-friendly QoS, and shared-buffer policies that don't penalise out-of-order delivery — the conditions UEC NICs need to operate efficiently.
TH4 / TH5 silicon
Runs on the same Broadcom Tomahawk 4 (25.6T) and Tomahawk 5 (51.2T) platforms used for current RoCEv2 fabrics. No forklift upgrade required.
RoCEv2 + UEC coexistence
UEC and RoCEv2 traffic share the same fabric on different priorities. Migrate clusters incrementally as UEC-capable NICs deploy.
UEC-aware observability
Per-path utilisation, spray-friendly buffer counters, and reorder-tolerance metrics streamed over gNMI for closed-loop tuning during cluster bring-up.
Vendor-neutral path
UEC is the open alternative to InfiniBand. Pairing UEC-compliant OcNOS-DC with open hardware means no single vendor owns the AI fabric — which is the whole point.
UEC vs RoCEv2 vs InfiniBand — where each fits
- RoCEv2 today. Production-grade for clusters built right now. OcNOS-DC ships pre-tuned RoCEv2 on TH4 / TH5 — see the RoCEv2 deep-dive. Most fabrics in production in 2026 are RoCEv2.
- UEC tomorrow. The path forward for scale-out clusters as UEC NICs ship in volume. OcNOS provides the fabric layer; cluster owners pick the NIC vendor.
- InfiniBand. Single-vendor performance lock-in. Different cabling, different management, separate ecosystem. UEC closes the performance gap on a standards-based, multi-vendor Ethernet fabric.
- Co-existence is the default. Real fabrics carry RoCEv2 traffic from existing clusters and UEC traffic from the next-gen build alongside each other. OcNOS-DC supports both on the same hardware.
- Forward-looking GLB. The OcNOS 7.1 Global Load Balancing path-quality plane is being designed to interoperate with UEC signalling as the spec matures — investments in OcNOS-DC today carry forward.