Application Note

SmartNIC, Local Ports Retransmission Feature

Learn how Daqscribe’s Ethernet recorders use Local Port Retransmission to resend packets via SmartNICs with minimal delay, bypassing the host CPU.

September 3, 2025ethernetfpgarecording

Diagram of FPGA inline capture with local retransmission, showing a Daqscribe recorder passing traffic between a DUT and a RoCE endpoint while recording

Overview

This technical note explains the Local Port Retransmission feature in Daqscribe’s Ethernet recorder systems. Local port retransmission, in networking, refers to a SmartNIC resending received network traffic on the same device without involving the host CPU. For instance, a SmartNIC with local retransmission can receive packets on one port and then forward them out on another port on the same device, with minimal delay.

Purpose

Local Port Retransmission enables the real-time forwarding, loop-through, or mirroring of network traffic. This feature is handy in applications like:

Inline traffic recording and monitoring
Load balancing: Spreading network traffic across multiple servers by retransmitting packets from a monitoring device to various servers
Mirroring: Duplicating network traffic to multiple destinations, such as security analysis tools
TAP (Test Access Point): Network troubleshooting and diagnostics

Functional Description

The FPGA-based SmartNIC used in Daqscribe Recorders supports hardware-level retransmission. Packets arriving on one port can be retransmitted on the same or another port with extremely low latency and without CPU intervention. The retransmission process is handled entirely within the NIC’s FPGA logic.

Key Characteristics:

Operates at the full data rate of up to 100 Gbps per port
Low-latency forwarding at the microsecond level
Nanosecond-level timing accuracy maintained
Configurable behavior for each port
Deterministic performance with minimal jitter

Use Cases for Local Port Retransmission

Use Case	Description
Inline Tap Mode	Record and retransmit live traffic without interrupting data flow
Real-Time IDS/IPS	Deliver traffic to an intrusion detections system (IDS) or intrusion prevention system (IPS) while simultaneously recording
Replay Validation	Inject recorded or live traffic back onto the network for testing

Application Example: “Local Retransmit of RoCE v2”

Switch-Free Configuration Options

The following configurations enable full observability without relying on mirrored traffic from a switch fabric.

Diagram of a DUT and RoCE endpoint linked directly through Daqscribe FPGA capture ports, with all traffic captured and retransmitted inline without an external switch — Figure 1: FPGA Inline Capture with Local Retransmission

The DUT and RoCE endpoint are linked directly via the Daqscribe FPGA capture ports.
All traffic is captured and retransmitted instantly – no external switch needed.

Diagram of a SmartNIC in the Daqscribe system establishing a queue pair with the DUT, with all RDMA traffic routed through the FPGA capture ports and logged — Figure 2: FPGA Capture with Local NIC Participation

Adds a SmartNIC to the Daqscribe system to actively establish a queue pair with the DUT.
All traffic, including NIC-originated RDMA exchanges, is routed through the FPGA capture ports and logged.

Diagram of a software-defined radio and data processing station connected through Daqscribe FPGA capture ports, with traffic captured, retransmitted, and recorded — Figure 3: Software Defined Radio (SDR) Use-case

The DUT/SDR and Data Processing Station endpoints are directly connected via the Daqscribe FPGA capture ports.
All traffic is captured, retransmitted through FPGA capture ports, and recorded.

Diagram of two networks connected through Daqscribe FPGA capture ports while IDS/IPS software analyzes the inline traffic during full-speed recording — Figure 4: FPGA Capture IDS/IPS Use-case

Two networks are designated as endpoints and are connected directly through the Daqscribe FPGA capture ports.
Suricata and Zeek are examples of IDS or IPS software running on a Daqscribe Recorder, analyzing in-line network traffic while recording at full speed.

Configuration

Local Port Retransmission can be configured through:

Via system configuration scripts (file format) or from the GUI (future)

Example Port Mapping:

Port 0 → Port 1 (bidirectional pass-through)
Port 2 → Port 3 (loop-back testing)

System Requirements

Daqscribe Recorders featuring SmartNIC technology with firmware supporting local port retransmission.
Proper cooling and power supply for full-speed operation.

Notes & Considerations

Retransmission happens at Layer 2 and does not involve protocol stack inspection.
Filtering, timestamping, and other analytics remain supported simultaneously.

Support

For technical support or questions related to this feature, contact us!

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