ESXi TCP Receive Buffer Autoscaling Disabled for NFC Connections on Port 902

Products

• VMware vSphere ESXi

Issue/Introduction

• Network File Copy (NFC) and NBDSSL data transfer operations over TCP port 902 experience latency during large virtual disk backup and restore workflows.
• Packet captures indicate that the TCP receive window size remains locked (e.g., window size scaling factor fixed at 512) and does not autoscale dynamically.
• ESXi advanced TCP/IP autoscaling parameters (/Net/TcpipAutoScaleRcvBufSize, /Net/TcpipAutoScaleSndBufSize) are set to maximum values.
• /var/log/hostd.log log file shows messages.
  

  Hostd [2101645]: [Originator@6876 sub=Libs] Optimize socket buffer size.
  Hostd [2101645]: [Originator@6876 sub=Libs] NfcNet_OptimizeSocketBuffer: Setting buffer size using RTT=5 ms BANDWIDTH=134000 bytes/ms BUFFER_SIZE=654KB
  Hostd [2101645]: [Originator@6876 sub=Libs] [NFC INFO] NfcLog ConnectionSetupInfo: server TCP Socket buffer sizes receive: 670000, send: 670000
  Hostd [2101645]: [Originator@6876 sub=Nfcsvc] Peer Address: <IP>

Environment

VMware ESXi 8.0 Update 3
VMware Cloud Foundation 5.2

Cause

Autoscaling support is not added to NFC.

• The receive buffer clears the socket buffer autosize which disable TCP window autoscaling for that socket.
• On ESXi 8.0U3 the NFC socket buffer calculates 654KB based on and expected 1Gbps bandwidth.

Resolution

NFC bandwidth parameters and TCP socket buffer auto-tuning were introduced from 9.1.0