# Userspace Datapath - Checksum Offloading

This document explains the internals of Open vSwitch support for checksum offloading in the userspace datapath.

## Design

Open vSwitch strives to forward packets as they arrive regardless of whether the checksum is correct or not. OVS is not responsible for fixing external checksum issues.

The interface (internally referred to as a netdev) can set flags indicating whether each packet’s checksum is good or bad upon receipt. If this flag is not set, OVS will consider the validity of the packet’s checksum to be unknown.

OVS will not re-calculate or update the packet’s checksum if the checksum is already known to be correct, known to be explicitly incorrect, or destined for an egress interface that will recalculate the checksum anyways.

If OVS does invalidate the checksum, and the packet ingresses the datapath with a checksum that is not known to be incorrect, OVS postpones checksum updates until the packet egresses the datapath. This recalculation can either be performed by OVS or, be offloaded onto the NIC if the egress NIC supports checksum offloading.

When a packet egress the datapath, the packet flags and the egress interface flags are verified to make sure all required offload features to send out the packet are available on the egress interface. If not, the data path will fall back to equivalent software implementation.

## Interface (a.k.a. Netdev)

When the interface initiates, it should set the flags to tell the datapath which offload features are supported. For example, if the driver supports IP checksum offloading, then `netdev->ol_flags` should set the flag `NETDEV_TX_OFFLOAD_IPV4_CKSUM`.

## Rules

1. OVS should strive to forward all packets regardless of checksum.

2. OVS must not correct a known bad packet checksum.

3. Packet with only flag `DP_PACKET_OL_RX_IP_CKSUM_GOOD` means that the IP checksum is present in the packet and it is good.

4. Packet with only flag `DP_PACKET_OL_RX_IP_CKSUM_BAD` means that the IP checksum is present in the packet and it is bad. Extra care should be taken to not fix the packet during data path processing.

5. Packet with both `DP_PACKET_OL_RX_IP_CKSUM_GOOD` and `DP_PACKET_OL_RX_IP_CKSUM_BAD` means that the IP header is valid, but the checksum may not be set in the packet data. This is basically encountered with some virtual drivers, or after OVS modified the IPv4 header content of a not bad (“bad” as defined in 4)) packet.

6. Packet with neither `DP_PACKET_OL_RX_IP_CKSUM_GOOD` nor `DP_PACKET_OL_RX_IP_CKSUM_BAD` means that the IP header status is unknown. It may be a IPv4 packet, or not. It may have a valid IPv4 cheksum, or not. This situation is encountered with virtual drivers that provide no information about the IP header, and for IPv6 packets.