Frame Overhead Inconsistencies Within Drivers

https://bugs.dpdk.org/show_bug.cgi?id=1476

Bug ID: 1476
Summary: Frame Overhead Inconsistencies Within Drivers
Product: DPDK
Version: unspecified
Hardware: All
OS: All
Status: UNCONFIRMED
Severity: normal
Priority: Normal
Component: ethdev
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---

There are inconsistencies within DPDK network drivers regarding how it calculates Ethernet overhead. These calculations have meaningful impacts towards how we write test suites to assess the ethdev API, namely MTUs.

Network drivers base their implementations of rte_ethdev's set_mtu() on arbitrary Ethernet overhead constants specific to the individual network driver. For example, the file bnxt_ethdev.c assumes an Ethernet overhead of the standard 14 byte Ether header, CRC, and space for QinQ; i40e makes the same assumptions. Mellanox, however, assumes only for a 14 byte Ether header plus CRC, so the overall Ether overhead is different between NICs. DPDK drops or forwards packets based on the total frame size, not the L3 packet/MTU size. So, the result is that we can set the MTU the same way for different devices and see different behavior.

If we want to, for example, test MTUs without concern for individual behavior, then we need to discuss what the proper amount of assumed overhead is before we can consider it a bug.

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