In this blog post, we go into the trenches of the (Distributed) vSwitch with a focus on vSphere ESXi network IOChain. It is important to understand the core constructs of the vSphere networking layers for i.e. troubleshooting connectivity issues. In a second blog post on this topic, we will look closer into virtual network troubleshoot tooling.
IOChain
The vSphere ESXi network IOChain is a framework that provides the capability to insert functions into the network data-path regardless of the usage of a vSphere Standard Switch (VSS) or a vSphere Distributed Switch (VDS). The IOChain is a group of functions that provides connectivity between ports and the vSwitch. A port has two IOChains based on the direction to and from the vSwitch. Meaning each port in a set is associated with it an input and an output IOChain. This allows for a modular approach by only including optional elements in an IOChain as configured by the user.
Examples of optional elements in an IOChain are VLAN support, NIC teaming, and traffic shaping. Looking at the high-level components in an ESXi network IOChain, we differentiate between the port group, the vSwitch (VSS or VDS) and the uplink level.
Port group level
This is where an optional configured VLAN is interpreted by the VLAN filter, allowing for VLAN dot1q tags for your port group. The security settings Promiscuous mode, MAC address changes, and Forged transmits are also set at the port group level. The user can also optionally configure traffic shaping, either egress only when using a VSS or bi-directional traffic shaping when using a VDS.
vSwitch (VSS or VDS) level
Incoming packets at the vSwitch level are forwarded to their destination using the forwarding engine. Incoming packets at the vSwitch level are forwarded to their destination using the forwarding engine. The forwarding engine contains port information paired with MAC address information. It’s job is to send the traffic to its proper destination. That can be either a VM residing on the same ESXi host or an external host.
The teaming engine is responsible for balancing network packets over the uplink interfaces. The way it does so is depended on the chosen teaming configuration by the user. The traffic shaper module is added to the IOChain if enabled in the port group level.
Uplink level
At this level, the traffic sent from the vSwitch to an external host finds its way to the driver module. This is where all the hardware offloading is taking place. The Supported hardware offloading features depends strongly on the physical NIC in combination with a specific driver module. Typically supported hardware offloading functions that in NICs are TCP Segment Offload (TSO), Large Receive Offload (LRO) or Checksum Offload (CSO). Network overlay protocol offloading like with VXLAN and Geneve, as used in NSX-v and NSX-T respectively, are widely supported on modern NICs.
Next to hardware offloading, the buffer mechanisms come into play in the Uplink level. I.e., when processing a burst of network packets, ring buffers come into play. Finally, the bits transmit onto the DMA controller to be handled by the CPU and physical NIC onwards to the Ethernet fabric.
Standard vSwitch
The following diagram puts all components together to form the IO chain for vSphere networking using a standard vSwitch:
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