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The Ultra Ethernet Consortium was formed in July 2023 by AMD, Arista, Broadcom, Cisco, Eviden, HPE, Intel, Meta, Microsoft, and others. NVIDIA joined later. The goal: define a new transport protocol for AI/HPC workloads that runs over standard Ethernet physical layer but does not require PFC.

The UEC specification structure has four protocol components. These are not separate protocols layered on top of each other -- they are four functional blocks within a single coherent transport design:

UET (Ultra Ethernet Transport) -- the packet format and delivery model. Defines how data is segmented, how messages are packetized, and what the packet header looks like. UET is the thing that replaces the BTH (Base Transport Header) layer in a RoCEv2 packet.

Retransmit -- reliability without lossless fabric. Defines the selective acknowledgement (SACK) and retransmit mechanism that allows the transport to recover from packet loss without requiring the fabric to prevent it. This is the key structural difference from RoCEv2.

Congestion control -- ACK-based, not pause-based. Defines the congestion signal mechanism using acknowledgement packets and explicit signalling rather than PFC pause frames. The congestion signal is end-to-end (sender to receiver) not hop-by-hop (switch to switch).

Multipath -- native packet spraying. Defines how packets from a single message are sprayed across multiple paths, and how the receiver reassembles them correctly regardless of delivery order. Out-of-order delivery is a first-class design assumption, not an edge case.

UEC protocol stack -- four functional blocks within a single coherent transport
Application / NCCL
AllReduce, collective ops
RDMA Verbs APINEW
ibv_post_send / ibv_poll_cq -- unchanged from RoCEv2
Multipath (UEC component 4)NEW
Explicit path IDs, NPM feedback, round-robin spray
Congestion Control (UEC component 3)NEW
ECN CE -> ACK C-flag -> rate reduction -- 1 RTT loop
Retransmit (UEC component 2)NEW
SACK bitmask, per-message timer, NIC-level recovery
UET Transport (UEC component 1)NEW
20-byte header, Message ID + Packet Offset, DCID/SCID, UDP 4792
UDP / IPv4 / EthernetNEW
Unchanged. Same Ethernet physical layer as RoCEv2.
UET Transport (UEC component 1)
Replaces BTH (12 bytes) with UET header (20 bytes). Key fields: Message ID (32-bit, per-context), Packet Offset (32-bit byte offset within message), DCID/SCID (24-bit context IDs), OpCode (8-bit), Flags (ECN-capable, CE, solicited, first/last). UDP port 4792 (vs RoCEv2 4791).
UET header structure (20 bytes):
Ver(4b)
OpCode(8b)
Flags(4b)
DCID(24b)
SCID(24b)
Message ID(32b)
Pkt Offset(32b)
Payload Len(16b)
Ack/SACK(32b)
Compare: BTH (12 bytes): OpCode + PKey + QPN + PSN -- no offset, no ack
UDP port 4792 (UEC)vsUDP port 4791 (RoCEv2)Click a layer for detail

The UEC 1.0 specification was ratified in Q4 2024. As of March 2026, UEC 1.0 is the current published spec. The spec defines the protocol behaviour required of any UEC-compliant endpoint (NIC or DPU) and the fabric behaviour required of UEC-capable switches. It does not mandate specific silicon implementations.