Account for large send offload when interpreting an Azure VM packet capture

Source facts

Microsoft’s Azure VM networking guide warns that large send offload can produce large frames in packet captures that are mistaken for fragmentation or a larger MTU. With LSO, the adapter advertises a larger segment size to the host stack and then splits the resulting large packet according to its MTU. The capture interpretation must account for that processing. Microsoft Learn.

Applicability

Apply this check when analyzing captures taken on an Azure VM. Identify the capture location, interface, offload configuration and the actual question being investigated. Keep the observed capture size separate from an independently established path-MTU value.

DSE recommendation

DSE recommends recording offload settings with the capture evidence before recommending an MTU change. Ask the network owner to explain where segmentation occurs relative to the observation point. If another approved capture location is available, use it to compare the same test flow. Preserve the initial capture and settings rather than changing several performance controls at once.

Verification

Correlate the representative transfer across the available observations and compare it with the documented offload behavior. Check actual application success and any independently observed fragmentation or MTU errors. Label conclusions according to what was measured. If offload settings are changed for diagnosis, use an approved reversible test and restore the intended configuration afterward.

Official references

Microsoft Learn: TCP/IP performance tuning for Azure VMs.